HDSDR IF Panoramic Display with Conventional Receiver or Transceiver

HDSDR main page https://sites.google.com/site/g4zfqradio/installing-and-using-hdsdr

Notes about "Subharmonic" mixing.

Hardware

Sync Modes

I often refer to a Softrock SDR on this page but any SDR that will work with HDSDR may be used.

A simple crystal-controlled SDR like the Softrock Lite may be connected to the IF of a conventional receiver or transceiver to give a panoramic SDR display and all the other features of SDR. HDSDR and rig are connected by CAT so that either may be tuned, they both move together. With a transceiver you may click on a signal and transmit. Ham Radio Deluxe may be connected by DDE.

When the SDR adaptor has been installed and connected HDSDR and Omni-Rig Version 1 need to be configured so that HDSDR's display and tuning coincide with the associated equipment.

Omni-Rig needs to be set so HDSDR and the external rig talk to each other. It should just be a matter of connecting the rig's interface and configuring Omni-Rig to match the rig settings. Note the "Poll int" may be reduced, maybe to 100ms, for quicker response. A virtual COM port is often required eg COM-0-COM

Initially set the "RF front-end frequency options..."

For a tuneable IF receiver that is controlled by HDSDR:- Like below with just the nominal IF frequency of the external receiver entered (Normally with an offset, maybe 10000-15000HZ). With a tuneable SDR HDSDR will set the centre frequency if the DLL is selected, you may trim this if required.

With a crystal controlled IF receiver "IF Frequency" has no effect. The "Global Offset" has to be set exactly to compensate for the actual crystal frequency.

Note pressing "F7" then "R" on the keyboard will get you to the "RF front-end frequency options...". You may need this quite a number of times when trying to get it all absolutely precise. Do not use the "LO frequency calibration!"

There are four tuning options for the IF panadaptor. Start with "Full sync in both directions"

"Mirror RF Spectrum in general" In this application it seems to have the same effect as "Swap I and Q Channel for RX Input".

"Mirror RF Spectrum for Tune >=...Hz" is for some radios that use a low IF. I/Q will be reversed above and below the IF. The K2 and K3 are examples of this type of radio. Enter the first IF frequency here.

"Global Offset" may be left at zero. It may be set later. It's main function is to avoid using the SDR's centre frequency for receiving. It represents the offset of the SDR LO and the SDR tuned frequency. It may be fine-tuned so HDSDR and the radio frequency exactly coincide, although the better way is to adjust the "Local Oscillator" frequency. The effect is the same.

As an example:- a RTL dongle may well need this to be around 50000 to avoid the noise that is often seen around the centre. To compensate for this figure 50000 must be added to the "Local Oscillator" frequency. Soundcard SDRs may be used around a setting of 10000.

My way of calibrating the frequency.

When CAT control is established the first stage must be to check that the "Swap IQ channel for RX Input" is correctly set.

Tune to an amateur band on the conventional receiver but listen to the audio from the SDR. Tune in several SSB stations with this receiver if the sideband heard on the SDR is not correct for the selected band (LSB for 40 metres, for example) then tick "Swap IQ channel for RX Input" .

The tuning will not match between the receiver and the SDR so the tuned signal will only be heard on the SDR.

"Swap I and Q Channel for RX Input" and "Mirror RF Spectrum in general" will seem to have the same effect but for a few rigs the sideband setting will change above a certain frequency. You will need to set that frequency in "Mirror RF Spectrum for Tune", see above.

My method for setting the various offsets is to tune to a strong, steady AM station of known, preferably accurate, frequency. This time tune to the frequency on the external receiver.

WWV, or one of the major shortwave broadcasters may be used. I have used a local BBC medium wave station on 999KHz. Like all major broadcasters the BBC usually maintains very good frequency accuracy.

Choose a station on it's own with no other strong signal close by so there is no confusion. First set the mode to CW. You will see something like that below. The station on 999KHz (999000Hz) is seen at 1008177Hz when you hover your cursor over it's carrier. 1008177-999000Hz = 9177Hz. If your SDR oscillator is lower than the IF frequency this figure will be negative.

Enter this figure into the "Offset" box.

You may first approximately calculate this figure. It is the difference between the rig's IF and the Local Oscillator of the SDR. It will be a negative value if the SDR's LO is lower than the IF. In my example the SDR frequency is about 10KHz higher than the IF.

Now the AM station should be perfectly tuned, if a little out then tweak the figure a little.

I recommend making a note of settings once you get near. If the figures get lost or you make a mistake you will not have to start again. Or, if you have setup a profile then close HDSDR and open it again as you progress. Settings are not saved until it is closed.

You may wish to make the "Global Offset" a whole number. say 10000. In my example above add 823 to "Global Offset" add 823 to "Local Oscillator" to compensate.

Now set to LSB, if tuning is not zero-beat check the difference, again reading from the cursor position.

Here 999000Hz appears at 1001328Hz, difference 2328Hz.

Enter this into the "LSB" box.

Repeat on other modes you will use, enter any offset you find. I found a negative offset for USB, no offset for the other modes.

ECSS mode does not work with HDSDR used in IF mode with a fixed SDR.

These figures (except the 9177 Offset:) are likely to be similar for other FT840s.

Another way of reading the offset for each mode is to select each mode and tune HDSDR so the red line is exactly coincident with the AM carrier. Then read what the "Tune" frequency indicates. This may produce more accurate results but I'd recommend reading the cursor value first. Make a note of the readings then try the tuning method. This choice really depends on how familiar you are with tuning HDSDR.

(True frequency of signal is 999000) Here the USB offset is 998310-999000 = -690 I suspect that absolute accuracy will not be achieved with my FT840, the minimum tuning step is 10Hz. And my FT847 can only be tuned in 10Hz steps with CAT.

Finally setup image rejection. https://sites.google.com/site/g4zfqradio/hdsdr-iq-balance

Using the First IF for full Range of an SDR.

The FT840 like many amateur rigs has a 47.05MHz crystal roofing filter immediately after the first mixer. The only way to get a wide panadaptor display is to make the tap before this filter. Below a 96KHz span.

I have used about a 20KHz offset but with "subharmonic mixing" I think a much lower offset may be used as there is no signal present to feedback into the receiver's IF.

Subharmonic mixing is used with Softrocks as they will not work using an oscillator at four times the received frequency of 47.05MHz.

Here I have used a Softrock like the Ensemble with a 47MHz BPF and an Si570 programmable oscillator.

The Softrock uses an FST3253 switch as a mixer. For HF this is normally switched at a frequency close to the signal frequency. But this type of mixer also responds to signals at odd multiples of the switching frequency.

So, if the Si570 oscillator is set to, say, 62.76MHz it will be divided by 4 = 15.69MHz and sent to the mixer. Signals close to three times this frequency will be received. 15.69 x 3 = 47.07MHz. This is called subharmonic mixing and will give an offset of about 20KHz. Using this technique I think much smaller offsets will be satisfactory, always avoiding the area close to the centre.

Subharmonic mixing using switching at 1/3 of the receive frequency samples at 1/3 of the rate so the recovered signal voltage is 1/3 (9.54dB) less than mixing with the LO at signal frequency. A buffer amplifier will most likely be required with a Softrock.

HARDWARE

For details please search the web, many common rigs have been adapted. Here is a detailed look at one for the K3. http://www.cliftonlaboratories.com/elecraft_k3_and_panadapters.htm (This link is dead, copy it and enter here http://web.archive.org/ , maybe make a donation?) For the maximum bandwidth the IF usually needs to be taken from just after the first mixer. Choose a soundcard SDR's oscillator frequency carefully. It is likely to feedback into the rig and although it must be close in frequency to the rig's IF it must not fall into the passband of any of the rig's filters. (But look at the bottom for "subharmonic mixing" a way of using the Softrock at 50MHz, maybe up to 70MHz)

IF Tap.

An increasing number of modern receivers have an IQ output. The KX3 is one, look here for the setup with HDSDR http://hdsdr.de/Elecraft-KX3_with_HDSDR.pdf

At the bottom of this page https://sites.google.com/site/g4zfqradio/ensemble_mods I give an idea of my home-made adaptors for my FT840. See my notes at the top of this page.

The G4HUP adaptor is now sold by SDR Kits https://www.sdr-kits.net/Panoramic-Adaptor-Tap-Boards

Here https://kd2c.com/hi-z-tap-boards is an adaptor supplier.

Note for a first IF tap this will need to be very close to the tap point, long coax is likely to detune the IF transformers.

Receiver.

For the 2nd IF tap a simple Softrock Lite may be used.

For the higher 1st IF a Funcube dongle might be used. A RTL dongle may give acceptable results. Or, for the experimenter maybe a modified Softock Lite and subharmonic mixing.

For my reference! IF 47050000 Global Offset 8220 CFGSR 15.693 (=62.772MHz)(LO=47.079MHz) Centre freq is 8220Hz below tuned freq.

With tuneable LO the Global Offset/Actual LO affect tuning accuracy, are interdependent. Changing LO moves tuning in respect of display centre.

Finally setup image rejection. https://sites.google.com/site/g4zfqradio/hdsdr-iq-balance

With Ham Radio deluxe (HRD)

Mike, in the FCD group uses a FCDPro+ to take a signal from his transceiver so HDSDR will act as a panadaptor. His transceiver, HRD and HDSDR all work together and all synchronise. I used his description here as a basis.

Set up HRD to control your transceiver via serial CAT. Enable DDE in HRD.

In HDSDR - RF Front-end Frequency options change the SDR on IF output to 'manually'. Enter your transceiver's IF frequency in Hz.

The Sync Mode you use depends on your preference. Note "Independent" enables dual receive.

(If you are not using the IF output but HDSDR is setup as a normal receiver then just the DDE will need to be activated.)

Then under Options:- DDE to HDSDR set HRD Tune and Manual connect -

it should say "Status: connection OK" in green.

Now the transceiver should tune via it's own dial or HRD or HDSDR and all should be in sync. Plus the FCD is still locked on the transceiver's IF.

Other settings will be similar to those in the general section above. Use the link "My way of calibrating the frequency." at the top of this page.

Choosing the frequency of the local oscillator (LO).

Fixed frequency IF receivers like the Softrock Lite IF range use a cheaply available stock crystal. You have no choice. In some cases the "Global Offset" may be 20,000Hz or more.

If you have a choice, consider carefully.

It is not normally advisable to include the LO frequency within the tuned bandwidth an offset of about 10KHz is desirable.

If you are using the SDR receiver in normal mode then there will be a signal at the LO position, right in the middle of the receiver's IF passband. This could seriously affect the operation of the receiver. So a LO offset of, maybe 10-15KHz will be required.

If you use the "subharmonic" technique this signal will not be present but you will normally still require an offset of about 10KHz to keep the receive passband away from the centre.

My HDSDR main page https://sites.google.com/site/g4zfqradio/installing-and-using-hdsdr

G4ZFQ April 2013

alan4alan at googlemail com

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