2. LibreSDR

This page showcases measured performance for a LibreSDR I bought from OpenSourceSDRlab on Aliexpress.

The original seller link would be: https://opensourcesdrlab.com/products/libresdr-zynqsdr-ad9363-zynq7020

The package received contains:

  • 1x SDR device in antistatic Bag

  • 2x USB-C cables

  • 1x ethernet cable

  • 1x Allen key

  • 1x USB-A to USB-C adapter

  • 4x antennas

  • 1x blank 32GB micro-SD card

_images/opensdr_box.jpg

Box content

It can be bought in a variety of places such as Ebay, Aliexpress, Banggood… and of course the official seller’s own website.

2.1. Hardware

Feature

Description

FPGA

XC7020-2CLG400I with 2-core Cortex A9

RAM

1GB 32bit wide DDR3 1066MHz

Flash

32MB QSPI, also support micro SD card

RFIC

AD9363 (Can be “hacked” to AD9361 or AD9364) with 2 Transmit + 2 Receive channels

ADC/DAC resolution

12 bits

Max Sampling rate

61.44 Msps

Max bandwidth

20 MHz (AD9363 mode) or 56 MHz (AD9364/61 mode)

Frequency range

325~3800 MHz (AD9363 mode) or 70~6000MHz (AD9364/61 mode)

Clock

VCTCXO 40MHZ 0.5ppm, with external clock input

Connections

USB 2.0 OTG + USB debug (Serial) + Gigabit ethernet. USB connectors are Type C

As far as I know the current/latest PCB revision is “rev5”. Schematics can be found here

_images/zynqsdr_pcb.png

LibreSDR typical PCB

Caution

There are multiple sources/manufacturers, some official and some not. The alleged original designer “jiskra” mentions on this thread that some knock-offs exist that use 2nd-hand ADI chips, i.e. chips that were pulled from other products, reballed, and reused.

He mentions that the devices provided by OpenSourceSDRlab should be original ones

Users have reported that some versions seem to have no stiching vias. This is the case of the Hamgeek version for example. The vias appear to be there if we look closely, but the surface coating (resist, finish) seems te be very thick and covering them to the point where we do not see them anymore.

There are two main casing variants: one with “vertical” stripes in the aluminium shell and one with “horizontal” stripes. This can be seen here. Markings/engraving also exists in different flavours. The case marking of my device matches the one shown by sq6emm here.

My device has the marking “LibresDR” (lowercase “s”) and stiching vias are clearly visible on the PCB:

_images/opensdr_pcb.png

Casing and PCB of OpenSourceSDRLab device

Performance data given below is for the default configuration of my device (as I received it):

  • AD9363 mode (325-3800MHz / max BW 20MHz)

  • 1RX + 1TX

All tests:

  • Are conducted using the USB connection. This would NOT affect high-bandwidth measurement as IQ data was taken as “one-shot”, not continuous streaming.

  • Have setup cable loss deembedded from the measurement: the reference plane is the device’s SMA connector

Note

I have also tested the Hamgeek version but my Hamgeek device, acquired through Banggood, seems to have an issue. It works but some key metrics appear to be really bad. I will give them the benefit of the doubt and assume I just got a faulty device, and therefore will not report what I measured on this website.

2.2. Firmware

My devices came with a default firmware dubbed “v0.37-dirty”.

The firmware:

  • Is in “AD9363” mode, ie limited to the standard frequency range of 325-3800MHz / 20MHz bandwidth

    _images/linux_default_cfg.png

    Default RF mode

  • Is configured in 1TX-1RX only (so TX2 and RX2 port unusable)

  • Has ethernet enabled by default (some forums seem to say this is not the case but it was on mine):

    _images/linux_ethernet_config.png

    Ethernet configuration

    Note

    At the time of writing I have not tested ethernet yet, only USB

2.3. Typical test setup

The typical test setup is simple: the port under test is connected to the test instrument with an RF cable, and a 6dB attenuator is placed at the device’s SMA connector to tame any mismatch introduced by the RF cable.

_images/setup_typical.svg

Typical test setup

Setup cable loss has been characterised and is removed from the test results.

2.4. Port impedance

Test conditions:

Configuration

RF port

TX1 or RX1 port enabled
TX test: no active transmission

Sampling rate

1Msps

Tx Gain

0dB

Rx Gain

70dB

LO

1GHz

Parameter sweep

None

The TX or RX port is connected to a VNA and S11 is measured

_images/setup_impedance.svg

Port impedance setup

Test results:

From the plots we can see that

  • RX S11 looks OK between 800 - 2500 MHz

  • TX S11 looks OK between 250 - 2000 MHz

2.5. RX tests

2.5.1. Rx Frequency response

This test measures the frequency response of the receive chain

Caution

There are 3 frequency ranges defined in the Analog Devices documentation, each with a different RX gain table:

RX LO

Min GAIN

Max Gain

70 … 1300 MHz

-1 dB

73dB

1300 … 4000 MHz

-3 dB

71 dB

4000 … 6000 MHz

-10 dB

62 dB

Test conditions:

Configuration

RF port

RX1

Sampling rate

1.024 Msps

SG level

-15/-20 dBFs @ gain = 0dB

Tone frequency

LO+100kHz

Rx Gain

0 dB

Rx buffer size

2048

Parameter sweep

Rx Gain, LO frequency

Initial condition:

  1. The signal generator is set to transmit a tone at LO+100kHz

  2. The signal generator level is set to get circa -20dBFs at the receiver with a gain of 0dB (ie very strong but not so strong that we saturate the rx)

Sweep:

  1. At the start of each new frequency range: The dBm scale is calibrated using the 1st frequency of the active frequency range

  2. Sweep frequency and at each step the RX level in dBm is measured, using RSSI

Note

The final result is normalised to the level of the very 1st frequency

Test results:

Only two gains were measured: min and max to check if there was any dependency.

  • The frequency response looks very similar with both gains

  • There is significant rolloff after ~2500 MHz, consistant with the S11 response

The response roughly tracks the RX S11 where

  • S11 is good until 2500 MHz, and frequency response is somewhat flat.

  • Then S11 degrades and reaches its worst point at around 3500 MHz, which corresponds to the lowest point on the frequency response.

  • Past 3500 MHz S11 improves slightly again and the frequency response starts going up slightly again.

2.5.2. Rx Gain

This test measures the Gain of the receive chain. What we report is the gain error: it shows us gain linearity at the same time.

Test conditions:

Configuration

RF port

RX1

Sampling rate

1.024 Msps

SG level

-80dBFs @ gain = 0dB

Tone frequency

LO+100kHz

Rx buffer size

2048

Parameter sweep

Rx Gain, LO frequency

Initial condition:

  1. The signal generator is set to transmit a tone at LO+100kHz

  2. The signal generator level is set to get circa -80dBFs at the receiver with a gain of 0dB

Sweep:

  1. Sweep gain, RX level in dBFs is measured using an FFT at each step

  2. The Gain error for each step is found by comparing level measured at Gain G vs level measured at Gain 0dB

Note

The final result is normalised to the gain of 0dB

Test results:

  • The Gain Error remains relatively small across the whole gain range and across frequency.

  • There is a global tendancy for gain error to increase (somewhat) linearly with requested gain: this means that the gain is not quite linear.

2.5.3. Rx Bandwidth filter response

This test measures the frequency response of the baseband filter that sets the bandwidth

Test conditions:

Configuration

RF port

RX1

LO

1000 MHz

SG level

-20dBFs @ gain = 0dB

Tone frequency

LO+100kHz

Rx buffer size

2048

Sampling rate

min(3*BW, 30) MHz

Parameter sweep

Rx Bandwidth

Initial condition:

  1. The signal generator is set to transmit a tone at LO+100kHz

  2. The signal generator level is set to get circa -20dBFs at the receiver with a gain of 0dB

Sweep:

  1. While the signal generator frequency is kept constant, the RX LO is swept across the entire bandwidth of the filter (and a little more)

  2. Measurement is done in dBFs using an FFT at each step

  3. The FFT bin where the tone is expected is computed, and tone level measured

This methodology allows us to get a result that is independent of front-end loss

Note

The final result is normalised to the center frequency of the filter

Test results:

  • We see an interesting ripple in the filter response. I have checked manually and it is real, not a measurement issue. On wide bandwidths the passband ripple gets quite large, around 1.5dB

  • Filters at 250 kHz and 300 kHz BW look very similar and are too wide

2.5.4. Rx Noise figure

This test measures the noise figure of the entire receive chain

Test conditions:

Configuration

RF port

RX1

LO

1000 MHz

SG level

-80dBFs @ gain = 0 dB

Tone frequency

LO+100kHz

Rx buffer size

2048

Rx Gain

70 dB

Parameter sweep

Rx Bandwidth, Rx LO

By lack of noise source or any specialised equipment, I used the gain method to measure NF. It is theoretically possible since the maximum RX Gain is very high (~70dB), assuming thermal noise dominates. It is a “quick and dirty” method but should yield acceptable results.

The idea is to measure noise power while RX is terminated with 50R at maximum gain, and apply the formula:

\[NF = P_{noise,dBm} - (-174_{dBm/Hz} + 10*log_{10}(BW_{Hz}) + Gain_{dB})\]

Initial condition:

  1. The signal generator is initially set to transmit a tone at frequency RX LO + 100kHz

  2. The signal generator level is set to get circa -80dBFs at the receiver with a gain of 0dB

Sweep: For each frequency,

  1. (Max) Gain and dBm scale are calibrated using the signal generator

  2. Once Gain and dBm scale are calibrated, the signal generator is turned off

  3. The total noise power in dBm at the receiver is measured using an FFT

  4. Finally the NF formula given above is applied

Note

The test has been run with only the RX active

Warning

This test was not run in shielded environment.

Test results:

The results are quite reasonable. For this type of device we should expect between 4-8 dB of Noise Figure, which is exactly what we get. Past 2500 MHz the NF degrades which is in line with S11/Frequency response.

Active TX:

If we activate the TX (turned on but not transmitting), degradation may appear where the TX chain produces spurs that couple on the RX. Below is an example with the TX LO set at 1GHz:

We notice some “discrete” degradations in NF, especially at 1 MHz and 5 MHz bandwidth. To avoid these:

  • Ideally don’t activate the TX at all (as you can only deactivate by rebooting the board)

  • Or set the TX LO as far away as possible from the RX LO

If you are actively using the TX at the same time then be aware it might degrade the NF

2.5.5. Rx Spurious emissions

Spurious emissions measure the noise at the RX input, in other words what leaks out of the receive port and seen by the outside world. For this we use a slightly different setup: the TX transmits into a 50R load while we observe the RX port with a spectrum analyser using a wideband scan.

_images/setup_trx_coupling.svg

RX spurious emissions setup

Three configurations are tested:

  1. RX only: The TX synthesizer is OFF

  2. RX + TX Idle: the TX synthesizer is ON but not transmitting

  3. RX + TX Active: the TX synthesizer is ON and transmitting: This 3rd case is effectively the TX to RX isolation at PCB level

Test conditions:

Configuration

RF port

TX1 / RX1

Sampling rate

5 Msps

TX Tone frequency

LO + 1 MHz

TX Tone level

0 dBFs

Tx Gain

0 dB (max gain)

Tx LO

613 MHz

Rx LO

513 MHz

Parameter sweep

Rx Gain

Note

Test frequencies are chosen to avoid falling on “remarkable” values such as integers of 100 MHz

Initial condition:

  1. RX lo is fixed at 513 MHz

  2. If enabled the TX Gain is set to max, tone offset +1 MHz and LO = 613 MHz

Sweep:

  1. Min and Max Rx Gains are tested

  2. For each gain a wideband scan is done at the analyser side.

Test results:

2.5.5.1. RX only

Given the levels involved, we see nothing. The few spurs that are present can be internal (eg 2052 MHz is the VCO frequency, 4104M MHz is VCO H2), others would be external noise picked up by cables.

2.5.5.2. TX Synth ON - Max TX Gain

The TX synthesizer is ON but not transmitting. Now we also see the VCO fundamental at \(4 \times 613 MHz = 2452 MHz\) and its H2. Very similar plots between the two gains although VCO H2 is lower at high gain.

Note

The plots above are measured at maximum TX Gain, lowering the TX Gain reduces the VCO spurs seen at the RX port so these plots represent the worst case

2.5.5.3. TX Active

Note

This last case looks at TX/RX RF coupling between the SMA and the chip boundary: it gives us the TX/RX isolation on the PCB

The transmitter is turned ON: On top the the VCO spurs, we the TX fundamental at 614 MHz and replicas of the transmit tone at approx. 1.84 GHz, 3 GHz, 4.3 GHz, 5.5 GHz… They remain much lower than the VCO spurs. The transmit fundamental sits at around -82 dBm which negligible.

The VCO frequency shows a much stronger spur at approx. -65 dBm, but it’s very far from the RX frequency so would not be an issue in a system as it could easily be filtered out. That is, unless you chose a TX frequency such that the VCO would fall near the RX…in which case you would have a serious problem.

Focusing on the region around the LO frequency, we have a TX tone at roughly +4 dBm (measured separately) and a spur on the RX side at -82 dBm or an isolation of 86 dB. While this figure will likely change with frequency, we would still be in the range of 80dB+ of isolation. It is quite clear from this result that the PCB will not be a limiting factor.

2.5.6. Rx Spurious reception

Spurious reception measures the noise at the RX output, in other words what leaks in the receive port and seen at the end of the receive chain. For this both TX and RX ports are terminated by 50R, and we perform FFTs on the RX while sweeping the RX LO.

_images/setup_50r_termination.svg

RX spurious reception setup

Three configurations are tested:

  1. RX only: The TX synthesizer is OFF

  2. RX + TX Idle: the TX synthesizer is ON but not transmitting

  3. RX + TX Active: the TX synthesizer is ON and transmitting

Test conditions:

Configuration

RF port

TX1 / RX1

Sampling rate

16.384 Msps

TX Tone frequency

LO + 1 MHz

TX Tone level

0 dBFs

Tx Gain

0 dB (max gain)

Tx LO

600 MHz

Rx FFT size

16384

Parameter sweep

Rx LO, Rx Gain

Initial condition:

  1. If enabled the TX Gain is set to max, tone offset +1 MHz and LO = 600 MHz

Sweep:

  1. Min and Max Rx Gains are tested

  2. For each gain the RX LO is swept from 350 MHz to 3800 MHz

  3. An FFT is performed every 5 MHz

  4. At each FFT (~16 MHz wide) we take 5 MHz only on the positive side to avoid noise around the LO on the left or filter rolloff on the right. We then aggregate the 5 MHz chunks to form a wideband spectrum plot.

Test results:

2.5.6.1. RX only

At 0dB Gain, the whole frequency range is completely quiet. At 70dB Gain we see a wide range of spurs which are actually also present at 0dB gain but hidden in the noise. The stronger spur at 3.145GHz, for example, can be seen on both.

At high gain the noise floor is lifted (visible “bumps”) at around 850 MHz, 1100 MHz, 1400 MHz and 2500 MHz

2.5.6.2. TX Synth ON - Max TX Gain

As soon as the transmitter synthesizer is turned on (max tx gain), we see a lot of spurs appearing. These will move around with the TX LO. We see a particularly strong VCO fundamental spur at 2.4 GHz (600 MHz x 4). At Maximum RX Gain it even reaches 0dBFs!

Hint

If you really care about RX performance, try to keep the TX fully turned OFF. One way is to put the RFIC in TDD mode, RX only.

2.5.6.3. TX Active

We see a few stronger replicas from the transmit tone but overall very similar to the previous case. Some spurs appear that were not visible before (eg Tx tone H3 at 1803 MHz) The AD9363 mostly sees its internal noise and does not seem overly affected by its output stage (that is there is no much difference between TX ACTIVE and IDLE as most contributions are made a soon as the TX synthesizer is enabled).

2.6. TX tests

Important

Unless otherwise specified, all TX tests are run with a tone at 0dBFs

2.6.1. Tx Frequency response

This test measures the frequency response of the transmit chain

Test conditions:

Configuration

RF port

TX1

Sampling rate

1.024 Msps

Tone frequency

LO+100kHz

Tone level

0 dBFs

Tx Gain

0 dB (max gain)

Parameter sweep

LO frequency

Initial condition:

  1. The device is set to transmit a tone at a frequency offset of +100kHz

  2. The device is set to transmit at maximum level and gain

Sweep:

  1. Frequency is swept and TX level in dBm is measured using a power meter at each step

Test results:

The output power varies from +4 dBm to -5 dBm, averaging around 0 dBm. It’s less than what we could expect compared to an ADALM-PLUTO, but it’s still quite decent. There is however, quite a bit of roll-off: there is 9dB of variation across the whole range, which is not very good.

The response roughly tracks the TX S11 where

  • S11 is good below 2000 MHz

  • Then S11 degrades and reaches its worst point at around 2200 MHz, which corresponds to a trough on the frequency response.

  • Then S11 improves again around 2700MHz and the frequency response also improves, and after that further degrades

2.6.2. Tx Gain

This test measures the Gain linearity of the transmit chain

Test conditions:

Configuration

Parameter sweep

RF port

TX1

Sampling rate

1.024 Msps

Tone frequency

LO+100kHz

Tone level

0 dBFs

LO frequency, Gain

Initial condition:

  1. The device is set to transmit a tone at a frequency offset of +100kHz

  2. The device is set to transmit at maximum level

  3. A reference measurement is taken at maximum gain

Sweep:

  1. Frequency and Gain are swept, and tx power measured at each step

  2. Gain is computed as difference between current measurement and reference measurement

Test results:

The plots show that the gain is very linear but only from from -70dB to -20dB. Above -20dB gain we start showing a little bit of compression, and below -70dB we can’t backoff enough, especially at higher frequencies.

2.6.3. Tx Filter bandwidth response

Not measured, see Rx Bandwidth filter response, I expect the response to be identical

2.6.4. Tx LO leakage and Image

This test measures the LO and Image while transmitting a test tone

Test conditions:

Configuration

RF port

TX1

Sampling rate

20 Msps

Tone frequency

LO + 1 MHz

Tone level

0 dBFs

Parameter sweep

LO frequency, Gain, Bw

Initial condition:

  1. The device is set to transmit a tone at a frequency offset of +1 MHz

  2. The device is set to transmit at maximum level

  3. The sdr sampling rate is set at 20 MHz

Sweep:

  1. Bw, Frequency and Gain are swept

  2. At each step LO and Image are measured using the analyser

Test results:

  • LO: Values match datasheet for both 0dB attenuation (-50dBc) and 40dB attenuation (-32dBc).

  • Image: There is a pattern of degradation at very low and very high frequencies, as well as a degration around -30dB of gain.

2.6.5. Tx Harmonics

This test measures the transmit harmonics only, relative to the fundamental

Test conditions:

Configuration

RF port

TX1

Sampling rate

1.024 Msps

Tone frequency

LO+100kHz

Tone level

0 dBFs

Tx Gain

0 dB (max gain)

Parameter sweep

LO Frequency

Initial condition:

  1. The device is set to transmit a tone at a frequency offset of +100kHz

  2. The device is set to transmit at maximum level and gain

Sweep:

  1. Frequency is swept, and harmonics are measured at each step. Fundamental is also measured

Note

my analyser could not measure higher than 6GHz

Test results:

Harmonics are reasonably low, the signal appears clean:

2.6.6. Tx Spurious emissions

This test performs a wideband scan of the transmit port in an effort to measure all possible spurs. The test is done at fixed LO.

Test conditions:

Configuration

RF port

TX1

Sampling rate

1.024 Msps

Tone frequency

LO+100kHz

LO frequency

513 MHz

Tone level

0 dBFs

Tx Gain

0 dB (max gain)

Parameter sweep

None

Initial condition:

  1. The device is set to transmit a tone at a frequency offset of +100kHz

  2. The device is set to transmit at maximum level and gain

Sweep:

  1. No parameter is swept, we perform a wideband scan on the analyser side

  2. Three configuration: Transmitter Active, Idle Max Gain, Idle Min Gain.

Test results:

2.6.6.1. TX synth ON, output OFF - Max Gain

The transmitter is ON but not actively transmitting. At Maximum gain we see mainly:

  • LO leakage at 513 MHz at -53 dBm

  • VCO leakage at 2052 MHz: the VCO runs at 4x LO which is quite typical. The VCO is leaking at -40 dBm, which is incredibly high

  • VCO H2 at 4.1 GHz

  • Harmonics of the LO

  • And other much smaller spurs

2.6.6.2. TX synth ON, output OFF - Min Gain

The transmitter is ON but not actively transmitting. At Minimum gain we see that all spurs are much lower. While the VCO spurs still stand out as the strongest, they are between 30-40dB lower compared to the maximum gain configuration.

Hint

This shows that if you must leave the TX ON but are not actively transmitting, set the gain to the minimum to minimize spurs

2.6.6.3. TX ACTIVE

Here we see what appears to be harmonics of the test tone and VCO spurs (and many other smaller spurs). However looking at the Tx Harmonics chapter, something does not match: The odd harmonics are very high, H3 for example sits at -10dBc, not at all what was previously measured.

Additionally changing the amplitude of the main tone does not reduce the “harmonics”, which makes no sense.

Zooming in manually around each harmonic explains what is happening (older measurement done at 500 MHz):

_images/opensdr_txharmonic_check.svg

TX spurs at LO = 500 MHz

Since the wanted tone is at \(\text{LO + 100 kHz}\), H2 will be at \(\text{2*LO + 200 kHz}\), H3 at \(\text{3*LO + 300 kHz}\) and so on. This is what was measured previously.

Observation 1:

Picture 4 (2 GHz) shows a strong spur at the center frequency: it’s the VCO fundamental spur, which can easily be mistaken for H4 on a wideband scan. This picture confirms H4 is really low, but the VCO leaks out like crazy. I’ve never seen VCO spurs leaking that much, that to me would point to a PCB layout (ie design, not production) issue.

Observation 2:

Pictures 3 and 5 show why H3 and H5 appear very high on the wideband scan. There are indeed very high spurs but they are NOT H3 and H5! On picture 3 the spur \(IM_a\) appears at \(\text{LO - 100 kHz}\) and on picture 5 the spur \(IM_b\) appears at \(\text{LO + 100 kHz}\): clearly not harmonics, but mixing products of the main tone with something else.

In this case the math is easy to work out: the main tone at 500.1 MHz is mixing with the VCO spur at 2 GHz and so we get two components:

  • One at \(\text{(2000 + 500.1) MHz = 2500.1 MHz}\): the spur \(IM_b\) we see on picture 5.

  • One at \(\text{(2000 - 500.1) MHz = 1499.9 MHz}\): the spur \(IM_a\) we see on picture 3

This explains why changing the amplitude of the main tone had no effect on the relative amplitude of the “apparent” harmonics: they were not harmonics.

Danger

This is real, not a measurement artifact. While the spectrum of the tone itself is quite clean, the fact that we have very high VCO leakage and mixing between VCO and tone produces a VERY dirty spectrum. \(IM_a\) will effectively contain 10% of the main tone’s power!

If you want to use this device for transmitting over the air you will need heavy filtering to stay compliant with your local regulations

Back of the envelope calculations:

  • Assume you add a PA to transmit at +20dBm

  • The IMa spur would be at +10dBm (assuming PA is perfectly linear and does not compress)

  • Let’s say spurs > 1GHz must be below -30dBm @ 1MHz MBW (ETSI/RED requirement) -> you want at least 40dB of rejection

  • Assuming a low-pass filter with a cutoff slightly above our wanted frequency, we’re going to want 40dB of rejection over ~1 octave to secure enough margin on IMa

  • The filter order to reach 40dB/octave would be ~7! (7*6dB/octave = 42dB)

This is simply not practical, and severely limits the output power one can hope to use.

2.6.7. Tx (baseband) intermodulation

Tx intermodulation was measured manually. The test is meant to show intermodulation products that appear at baseband level due to DAC and other baseband-related non-linearities.

If we transmit a tone at a certain offset \(+ \Delta f\) from the LO, we will of course expect an image at \(- \Delta f\). However, non-linearities in the baseband chain will also cause other spurs to appear. Specifically, in an IQ-based architecture, we will see odd-numbered spurs appearing on alternating side of the LO: if the wanted tone is at \(LO + \Delta f\), we will see a 3rd order IM product at \(LO - 3\Delta f\), 5th order at \(LO + 5\Delta f\), 7th order at \(LO - 7\Delta f\), and so on. Even-order spurs are normally significantly suppressed.

As their order increases, the amplitude of the spurs also decrease rapidly. Usually, only the 3rd and 5th order are prominent/can cause issues in a system

Test conditions:

Configuration

RF port

TX1

Sampling rate

10 Msps

Tone frequency

LO+500kHz

Tone level

-3 dBFs

Tx Gain

0 dB (max gain)

Tx LO

1000 MHz

Parameter sweep

None

Test results:

The spurs are illustrated in the manual measurement below. The span was limited to encompass up to the 5th order spur for better visibility. The highest spur in this example is the 3rd order product, approx. 55dB lower than the wanted tone (good rejection).

We also see two unknown spurs near -100 kHz and +300 kHz, I’m not quite sure what they are since they don’t fall on integers of 100 kHz. Maybe a clock or some fractional signal?

_images/opensdr_tx_bbim.svg

Baseband intermodulation

Unfortunately, since these intermodulation spurs occur at baseband it is not possible to filter them out if they pose a problem. The only way would be to decrease the tone level at the digital stage, before the tone is synthesized by the DAC.

2.6.8. Tx to Rx isolation

See Rx Spurious emissions, 3rd case.

2.7. Conclusion

The device works and has quite respectable performance, given the pricepoint the designer has done an amazing job. There are however two limitations I would highlight:

  1. Minor: The frequency response in both RX and TX could be improved.

  2. Major: TX spurious emissions are really bad owing to huge VCO leakage mixing with transmitted signal.