SDR TX IQ Image Indicator

Back in October 2015 Leif SM5BSZ suggested a simple untuned receiver that will indicate the degree of IQ imbalance in a SDR transmitter's output.

This was in the Yahoo Softrock group but did not get carried through properly when we changed to Groups.io

Required, simple RX as below, 96KHz soundcard to SDR program, best set for mono input or wideband audio monitor program maybe like SpecLab.

Yes, it works as Leif says:-

Hi Alan and All,

If one has only the Softrock and wants to use the transmitter there is a simple trick that requires the building of a a simple circuit:

Use a diode, 1N4148 or whatever, to rectify the antenna signal.

R1 has to be fairly large to not distort the signal due to the rectified current. If the signal is 1W, the current is 0.14 A at a 7 V (both RMS values) To have the distorted current 60 dB below the signal we have to make the current through the diode something like 0.14 mA which at 7 V requires R1 to be 50 k ohm.

We want to see audio up to 50 kHz so C1 should have an impedance of 50 k ohm at about 100 kHz. That means 68 pF or so.

C2 should pass low frequencies. With R2 = 100 k ohms C2 should be something like 0.1 uF. Then R3 could be 1 M ohm for 20 dB gain. The opamp must be one with FET input stages for a low noise at these high impedance levels. Maybe 4.7 pF in parallel with R3 would be nice to suppress RF.

Power up your Softrock in Tx mode and make it generate a carrier. In case the LO is at i.e. 14.100 MHz and your intended frequency is 14.115 MHz, the transmitted audio tone is at 15 kHz. Use another SDR program to monitor the output across R4. (R4=10k, C3=1uF) If the signal is perfect you would see nothing!!

If your image rejection is inadequate, assume only 40 dB you would see a strong signal on 30 kHz. The carrier at 7V would add to or subtract from the image at 70 mV giving 140 mV across R1. That is 1.4 V on the output and about full scale on the soundcard. It should be easy to bring the image below -100 dB if things are temperature stable.

The LO leak-through will appear at 15 kHz and audio harmonics will also appear at 15 kHz and all multiples of 15 kHz.

At 1 watt output I would consider an image 40-50dB below peak would be good enough, but Leif is a perfectionist.

My build

Obviously track broken under horizontal components and IC. Track relieved around pins 1, 5, 8 not connected.

I had intended the IC to be the other side of the board but made a mistake. TL081 but any equivalent would do.

Something similar could be made with wired components.

Connections Left, RF input from"T" connector on dummy load.

Right 12 volts and screened lead with 3.5mm audio jack. This can be a stereo jack with the tip and sleeve connected together for the soundcard input.

NOTE THIS DOES NOT CHECK THAT IQ SWAP IS CORRECT YOU COULD STILL BE ON THE WRONG FREQUENCY.

I much prefer a separate SDR receiver for this purpose but this will work once initial setup has been checked.

G4ZFQ February 2017