Direct Conversion RXs

There is no doubt that direct conversion receivers can be both low cost and effective. This page (linked from Hot Iron 110) has some interesting discussion. In the past, I have used direct conversion receivers in many designs.

See https://www.raynetrepair.us/circuit-design/some-practical-design-approaches.htm  .

Pipit 15m transceiver

This rig was designed and built many years ago. It spanned some impressive distances and worked well. The receiver was a direct conversion and the gain adjustment was before the mixer, so RF gain could be reduced easily which helped strong signal handling. No RX-TX offset was needed as this was just about right anyway. For quite a time it was the main rig used and every QSO felt like a million dollars! It was written up in GQRP SPRAT a very long time ago (30 years ago?). It is hard to get much more fun than using something this simple and working a long way regularly with it.

Before I had crystals for the CW part of the band my very first QSO was a USA SSB station that I called that was located near Chicago some 4000 miles away. I nearly fell off the chair when he replied.  After than I knew anything was possible. I really should rebuild it someday.

It crossed the Atlantic many times. I even used a 10/15m trap dipole (very low) occasionally and never used anything bigger than a low wire dipole. Output power was 800mW, quite enough I found for solid CW QSOs.

See https://sites.google.com/site/g3xbmqrp3/hf/15m_pipit .

Direct conversion ICs - CMX994

Since my days in mobile radio (now nearly 7 years ago) things have moved on.   CML now sell a VHF/UHF direct conversion IC covering 50-940MHz. See http://www.cmlmicro.com/Products/CMX_994_RF_Direct_Conversion_Receiver/?gclid=COrc7qiJ0sICFdLItAodLxEAuA . I wonder if any enterprising radio amateurs have used these yet? I have a feeling the package (ideal for SMA but not for home use) may inhibit its use in amateur projects?

http://www.cmlmi use) cro.com/api/i.aspx?CMX994Promotion.png;529;198

Phasing direct conversion receivers

What I think is quite an old paper turned up in my browsing today: http://www.norcalqrp.org/files/austinnc2030presentation.pdf . In it, Dan Tayloe N7VE describes how a direct conversion design with phasing to cancel out the unwanted sideband can be a truly excellent receiver. I have never tried this approach myself, but think it would be worth it for, for example, a WSPR receiver where the phase accuracy has only to be maintained over a 200Hz audio bandwidth. Over such a small audio bandwidth a very simple phase shift network would work.The same applies on TX where a very simple final frequency phasing TX could produce quite good sideband suppression with very few parts.

WISPY Schematic (more clearly)

For some reason when I put multiple images on a post it is not easily possible to see them clearly when clicked (at least with Firefox). So here is the schematic again. Clicking this image should bring up the full sized image clearly.

Hum fixed on the 28MHz WISPY WSPR RX

Rev B 28MHz WSPR receiver

Adding more audio decoupling around the oscillator and adding a common base FET RF preamp has totally fixed the common mode hum issues with my Polyakov mixer based DC receiver (WISPY) for 28MHz WSPR. Even running from a mains PSU I am getting good sensitivity and no hum. The board is working on the bench with no attempt at screening and appears totally fine. The oscillator buffer appears redundant, so has been removed.

Wispy Rev B (built using MeSquares)

One possible further mod is to make the input to the RF amp a series tuned circuit. This would be needed when the full transceiver version is made (back-to-back diodes would short the cap to ground on TX).  For now, I will leave the little RX running to see what I spot.

WISPY (Polyakov RX) - 1st spot

I rebuilt my 10m WSPR RX today (the rig has to be called Wispy) onto a tidier, smaller board using MeSquares and took a few steps to reduce the 50Hz common mode hum pickup, although there is still too much there and more work is needed. Someone suggested it was radiation of the 28MHz 2nd harmonic of the oscillator mixing with 50Hz in nearby PSUs being re-radiated and detected. Still unsure of the mechanism but there are lots of things to overcome this.

EA8FF on the Polyakov RX (note the 50Hz and harmonics lines locally generated)

However, the good news. The RX, even without a preamp, can detect down to an amazing -127dBm MDS, possibly lower but this is as low as my signal generator will allow and I did not try with an attenuator. My first reception of a WSPR signal was EA8FF at -18dB S/N. Remember this is a DSB RX so the noise figure is actually 3dB worse as it is picking up noise from both sidebands.

Next stage is to add a small preamp to see if this helps reduce the common mode 50Hz pick-up and increase the sensitivity another few dB.

Polyakov mixer and 50Hz hum

There is something strange about my Polyakov mixer based direct conversion receiver for 28.1246MHz WSPR at the moment.

On the bench, when connected to a 50 ohm signal generator or sitting on the bench without an antenna connected, the background noise is a quiet hiss. With an earpiece in a quiet room I can just hear a -125dBm signal, even without an RF preamp: it is a good, sensitive and simple design.

However, the moment I connect my outside halo antenna via a coax to the (unscreened) breadboard RX the 50Hz hum level is quite high, certainly some 40dB higher audio than the MDS signal. This is with the RX run from a small sealed lead acid battery. With a mains supply I might expect some hum, but what is going on? Why the high hum pickup?

My coax antenna feed passes through the loft and then outside passing plenty of mains cables, so I must be inducing some 50Hz pick-up on the coax outer which is coupling into the copper laminate breadboard ground plane. A tidy rebuild will be needed before I could put the RX into a screened box.

I do not recall having a similar problem with other DC designs I have tried (single balance diode pair, NE602, SBL1 etc.) when used in a similar lash-up breadboard.

Anyone any suggestions?