EMRFD Message Archive 4155
Message Date From Subject 4155 2010-02-02 13:46:01 Chris Pretty good Superhet as a test receiver I'm having some trouble aligning my T2 based transmitter. I don't have a good 40m receiver other than the R2 that I have also recently made and it shares the VFO with the T2. I tried aligning one with the other (using another VFO), but it's hard. Especially as I don't have switchable sidebands.
My plan is to make a simple superhet for test purposes. One of my main tests is to confirm that I have the T2 setup for LSB (and not DSB or USB).
I've picked Rick's 6 transistor "Pretty Good Superhet" as a circuit. It's in the archive of this group. But I have a couple of questions:
Do I need to use the Minicircuits T4-1? I presume not. I presume its a broadband transformer and splitter. Can I just wind 10T:10T:10T on type 43 material? My IF is 11MHz.
I'm not familiar with the crystal filter arrangement. What is the advantage of this over the ladder filter designs in EMRFD? What is the input impedance of this likely to be?
What is the best way to add switchable sidebands to this design, so that I can listen to both USB and LSB? Given I will use an IF of 11MHz, I presume I need to switch the BFO between 11.0027 and 10.9973 or similar.
any help appreciated,
G0KLA in W2 land.
4156 2010-02-02 14:54:40 victor Re: Pretty good Superhet as a test receiver A much better test receiver is to buy the softrock v9 lite receiver kit (with the automatic electronic switched input filters)from KB9YIG.
It is low priced ($56) and you get a full HF range SDR receiver which can be used as a spectrum analyzer with its spectrum screen width equal to the sampling rate of the PC audio card. I use it with Winrad software (freeware).
I use it constantly to see the audio modulation of my SSB transmitter. When the LO is frequency calibrated you can also easily measure carrier frequencies to 1 Hz precisi
4157 2010-02-02 14:56:05 Leon Re: Pretty good Superhet as a test receiver 4166 2010-02-02 23:11:33 drmail377 Re: Pretty good Superhet as a test receiver As Leon mentioned, you will need to increase the bandwidth of the crystal filter for SSB. You can to do this by decreasing the value of the capacitors in the filter. You can put a trimmer on the BFO to "pull" it, making it variable.
The Softrock Lite Version 9.0 receiver with switchable filters is a better solution provided you have a halfway decent sound card in your PC. However, good luck trying to buy one. Over the past few months it has become so difficult to by these radios from Tony (KB9YIG), I now consider the softrock project defunct. This may change in the future.
If you live in the US you might have better luck obtaining a softrock. I live in GMT+7 land. So when Tony puts a softrock kit
4167 2010-02-03 01:57:38 victor Re: Pretty good Superhet as a test receiver I was using the PC internal sound card with the Softrock receiver for quite a while. You just loose a few db's noise floor (not needed for measurements anyway) and you get a 48KHz frequency window.
You can build a SDR receiver yourself as there are numerous very simple examples in the web, and there is no real need for input filters if the receiver is used for measurements.
Victor - 4Z4ME
4175 2010-02-03 08:56:38 Chris Re: Pretty good Superhet as a test receiver Thanks David,
A couple of people have suggested using the Softrock, which is ironic. I have the Softrock Lite II for 40m and initially used this as the test receiver, with the transmitter into a dummy load. It turns out that my computer only has mono input to the sound card, so I'm not confident that I have USB/LSB with the correct orientation in the Softrock. I get a mirror of the signals above/below 7.056 with sidebands reversed.
I can see the signal well in the Softrock and agree it was a great way to tweak the I/Q channels on the T2, but did I tweak them for the right sideband? The Softrock Lite II only receives around 7.056, so I don't have other ssb signals to check the sideband setup against. I know, I know, this is a comedy of errors it seems ...
Anyway, I determined that checking a homebrew sideband phasing rig with a homebrew sideband phasing receiver introduced too many unknowns. Perhaps wrongly, I feel that I should be able to measure the frequency and bandpass of a crystal filter and measure the frequency of the BFO directly. I will then know what is going on and be able to test the transmitter more confidently.
Stop me someone if I am going down the wrong path.
G0KLA in W2
4180 2010-02-03 19:31:38 Pat Villani Re: Pretty good Superhet as a test receiver If all that's keeping you from using the softrock is the lack of stereo
input, have a look at DIAMOND XSTU21 USB 2 over at newegg.com (no
affiliation). It has a stereo mic/line input that you can use. Probably
your fastest and most cost effective solution.
4181 2010-02-03 19:31:44 svss_editor Re: Pretty good Superhet as a test receiver If you can tune a source up or down with confidence, would that help you get your bearings with your DSB softrock receiver? Worth a shot...
4184 2010-02-04 06:15:42 Tim Re: Pretty good Superhet as a test receiver What you need to align your transceiver, is a good clean oscillator and an ear.
The old fashioned 100kHz marker generator, for about half a century in any handbook, will get you a long, long way in satisfying you about oscillator stability, receive sideband rejection, and overall LO chain cleanliness.
What was a specialized and large piece of test equipment in the 50's and 60's, but today is just a single chip, is a frequency counter, and this is worthwhile too. BUT really nothing can beat even an unaligned marker generator, a receiver of even questionable alignment, and your ear.
If you want to measure a crystal filter, a frequency counter + a simple VXO or very limited range VFO is a wonderful tool. See EMRFD fig 7.27 or 7.32 and customize for the particular range of frequencies you need. I built fig 7.27 and extremely limited its range by tapping down
4193 2010-02-04 10:37:01 g0kla Re: Pretty good Superhet as a test receiver Yes Drew, that's basically what I did. I tuned the T2 CW signal up/down and determined that the carrier I could see below the Softrocks crystal frequency must be the "real" signal and not the image. When I transmited sideband the voice frequencies were ABOVE where the "real" T2 carrier had been. I figured that was wrong, so I reversed the IQ signals on my T2 and re-peaked the output.
Now the voice signals are BELOW where the T2 carrier would be (and for the image they are above). The LSB filter gives clean sounding audio below the carrier and for the image the USB filter works.
So my question is, was I right to reverse the IQ and re-peak for LSB? I had previously aligned this against the R2 before I packaged them with the same VFO in the same box and I thought I had it right. Maybe I did not and that is why no one responds to my calls..... Maybe its not my electrically shortened 40M dipole that's the problem.
Anyway, all this is very confusing for me because I don't really understand how the softrock works. I get that without IQ channels it is now a DSB zero IF receiver, but then can I only hear LSB below its carrier and USB above its carrier? I presume so. How does it work in normal operation then? Or is that the point, because you can't tune the softrock Lite II, it only detects LSB below its carrier and USB above its carrier. Because it's in the CW porti
4198 2010-02-04 22:16:51 Bill Wright Re: Pretty good Superhet as a test receiver Not defunt just demand exceeds supply. Tony is a mom and pop operation that is Tony and his wife.