On this page:-
Receive IQ balance adjustment.
Images and Centre Spike
Images are received and produced by soundcard SDRs. These appear exactly opposite the centre, LO (local oscillator) position. USB signals will have a LSB image. As an example, if the LO is at 7.100MHz, a LSB signal at 7.115MHz then the image will appear as a USB signal at 7.085MHz. The true signal is 15KHz above centre, the inverted image 15KHz below centre. The image should be considerably weaker so will only be noticed on strong signals. Some software automatically detects and minimises the image so that it is not noticed.
With a SDR transmitter it is always necessary to check and adjust with a receiver, no software has automatic adjustment.
Some soundcards produce a sharp spike in the centre, realise the centre represents zero frequency at the input of the soundcard, nothing can actually be received here. HDSDR has optional "DC Removal" to reduce this effect. Each side of centre the effects of ground loops might also be seen.
HDSDR is one of the few SDR programs that does not have automatic image rejection for receive. Some that do are not always reliable.
From an educational point of view learning to set IQ balance is a good thing. It teaches what image rejection does and prepares one for transmit checking and adjustment. No SDR program will automatically adjust transmit IQ. To avoid spurious signals from your transmitter you need to know where to look and what to do if adjustment is necessary.
Before receive adjustment make sure that the "Swap IQ" is set correctly. that the high frequencies are tuned on the right side. SSB signals will have an image but the strongest, real, signal must be the correct sideband. For example a USB signal should have a strong USB signal at it's correct frequency but it may have a lower strength LSB image exactly opposite the centre.
For optimum balance use the same soundcard frequency at all times.
Click on the signal on the HDSDR display and press Keyboard "C". Or just right click on the display. By default his will place the wanted signal exactly 10KHz above the centre frequency (LO). This ensures that wanted signals are always at the same audio frequency and variations in soundcard characteristics are minimised.
This is very desirable for transmit, keyboard "C" first. For receive it is not essential but if you do have image interference then keyboard "C" will help.
Below, the image reduction panel.
This is found Options - Calibration Settings - Input Channel Calibration for RX Tab.
The soundcard in use is an old Creative Soundblaster. While doing these examples I realised it is a poor choice for SDR.
When first started the image was only slightly lower in amplitude than the true signal.Note:- This is exceptional, but useful as a demonstration. In most cases the image will be 20-30dB down and easily reduced just by moving the sliders.
The adjustment must be made with the antenna you intend to use and at the frequency you are using.
On-air signals may be used but is best to provide a strong stable signal from a generator. Various methods may be used, a MFJ analiser, or low power transmitter into a dummy load, or maybe a stand-alone generator should provide enough signal to be picked up from the antenna without making a direct connection.
This Soundblaster card is one of the few soundcards that require a sample delay to be set.
After unsuccessfully trying to reduce the amplitude of the image at 1935KHz I clicked "Reset to 0" and experimented with the samples delay.
+1 sample delay reduced the amplitude.
In most cases you will see this without any sample delay.
Then, by careful manipulation of each of the "Raw" sliders in turn I was able to significantly reduce the image.
Further careful movement of the "Fine" sliders removed the signal entirely.
The cursor arrow below shows where it was.
Moving the generator across the band should show just the signal, maybe the image will become visible at low levels in places.
This old card proved very poor. Good balance was only possible in one place the image appeared strongly as the frequency was moved. This is unusual, most cards will maintain good balance.
There are a few that may not balance at all. Some laptops with Sigmatel sound have proved impossible.
Hiding the centre spike
"RX DC Removal" puts a notch in the centre and removes the visual and audible effect of the central spike.
Click "Mode" for 3 options including OFF, you may type into "Bandwidth" box to make the removal over a wider, or narrower frequency range. You could remove some bad hum but this will leave a big gap.
The prime reason this has been implemented is so that wide bandwidth modes may span the centre without the spike causing interference.
"RX DC Removal" is enabled on the pictures. The spike is suppressed although a small amount of hum pickup is still seen.
Normally it is best to avoid the centre, the notch is just that, there is a gap where nothing is received and there is often a degree of noise around the centre.
An external receiver is essential for checking any transmitter, even more so with SDR. They might transmit on the wrong frequency or on two at the same time! This RX could be another SDR, indeed the spectrum display makes adjustment easy and any other spurious emissions may be easily spotted. Do not overload the receiver.
Note that all SDR software will have TX IQ adjustments and will need to be checked in a similar fashion. You are setting up the software, not the hardware!
When in transmit mode several items change in HDSDR.
One is the Output Channel Calibration shown below.
This is found Options - Calibration Settings -
Another change are these sliders.
With HDSDR and your RXTX setup for transmitting FM click the TX button. Adjust the Output Power to a suitable level.
Using a receiver with just a small antenna, maybe a few cm of wire. Look for the transmitted signal. If tuned as in the above receive example you should find a strong signal on 1955KHz and a much weaker one on 1935KHz, the image.
In all cases if you have set the default offset by using "C" on the keyboard the image should appear 20KHz below the required signal. If the two are reversed then Swap I and Q for TX will need to be set.
The degree of imbalance may be roughly estimated by the external receiver's S meter. In many cases the image may already be low. If not it should be easy to reduce the image to a low level by setting the sliders on the Channel Adjustment Panel (TX). I am not aware of any soundcard that needs a sample delay set for transmit. The Soundblaster I used for these examples did not.
If the two signals are identical in amplitude then there is a fault in your hardware. One of the IQ signals may be missing or could be shorted together.
I have mentioned the default LO offset
(Later versions look slightly different.)
set LO <-> Tune Offset This adjusts the difference between the "LO" centre and the actual tuned frequency. The default is 10000 (10KHz) and is used when a frequency is typed in OR if tuned by a clicking the right mouse button. A figure of +10000 puts the tuned frequency 10KHz higher than the LO. Again by default the LO then remains fixed for further tuning in that range. Typing "C" on the keyboard moves the LO offset to the preset offset. The offset may be set to zero or any reasonable +/- value. The image will be this much below (or with a -ve value above) the LO.
Tune fixed to 'LO <-> Tune Offset' This causes the LO and tuned frequency to be fixed in relation to the "LO <-> Tune Offset". Whenever HDSDR is tuned the LO moves keeping the same offset. The LO offset is often important for maintaining good image rejection, particularly with sound card SDR transmitters. The amount of image rejection can vary a lot over a range of frequencies, best keep a constant LO offset. Zero, or close to zero offset may be used to keep any image or spurii near to the transmitted frequency. Using near zero offset exposes the receiver to any noise near the centre. Soundcard SDR can be made clear of noise by careful attention to ground loops and noise pickup.
Also note that "Frequency Manager" can save any odd LO and Tune frequency combinations.
Note that zero offset should not be used for AM mode. The carrier will be removed and audio will be badly distorted.
PSDR has IF shift for receive, the default may vary according to the version.
Most versions of PSDR remove this shift for transmit. The LO is changed to zero offset when transmitting, the image signal appears as the opposite sideband at the same frequency as the transmit signal. For the frequencies above if a 1955KHz USB signal is transmitted you need to look at 1955KHz LSB.
This will be the same should you choose zero offset in HDSDR but the LO does not change between receive and transmit.
Robby's page http://www.wb5rvz.com/sdr/RXTX_V6_3/image_rejection_hints.htm No mention of transmit, remember this is never automatic! You need to check this on all SDR transmitters.
My edited version of Alberto's manual to describe HDSDR's development from it. https://sites.google.com/site/g4zfqradio/winrad