DXTV challenge?

Sorry for the absence of postings last week but we've been on holiday. No ham gear with me, but I did spot several nice HF and VHF yagis on my travels.

Several of my blog readers were DXTV enthusiasts (like me a long time ago), so I wonder who will be the first to identify where we were on holiday by this testcard copied at 4am this morning on the hotel TV? No prizes for giving me the country, but interested to know how many out there know the answer and whether or not this card has ever been copied in the UK on Band 1 TV. This was on 625 line UHF.

A plug for my son's MS concert in Oxford, Oct 7th

My son Tim Lapthorn, who is a professional jazz pianist, is holding a concert in Oxford on Sunday Oct 7th to promote his new album. All proceeds will be donated to the MS Society. You are most welcome to attend and enjoy.

Tickets may be obtained from http://www.oxfordplayhouse.com/ticketsoxford/?event=17386. The lunchtime concert is in the Jacqueline du Pre Music Building, Oxford starting at 1pm.

As the publicity says:

"Benefit concert for Multiple Sclerosis Society. Tim Lapthorn and his trio will play material from the forthcoming album, Transport (Pathway Records) which includes brand new original material. The group will also play reworkings of jazz/folk standards. Please come along to this very special event to help support a great cause!"

See also http://www.st-hildas.ox.ac.uk/jdpevent/tim-lapthorn-trio-ms-fundraising-concert

Progress on the WISPY transmitter

This afternoon I managed to do a little more work on the WSPR transceiver for 10m, building the oscillator/x2 multiplier stage and the balanced mixer for the TX part using a couple of 1N4148 diodes and a T50-6 tuned circuit. Carrier suppression is around 30dB without much effort. Next stages will be to add a linear amplifier to bring the DSB output to around 1W (500mW pep SSB carrier). Although I could combine the same mixer on RX and TX, I may stick with the Polyakov RX mixer as this is working so well. 

I should have added that with a linear PA, the WISPY transceiver should also be perfect for PSK31 as the 14.060MHz crystal (when doubled) will certainly pull to the PSK31 10m sub-band.

New ICOM all-band multimode

Icom have announced that they will be introducing a new multi-mode, multi-band radio called the IC7100 which includes 70MHz at 50W on some versions. This is, to my knowledge, the first time one of the big manufacturers has properly included 4m in the design. I know at least one Yaesu rig was capable of working on the band, albeit with some compromises. The new Icom rig has a separated control head with a sloping front, which is ideal for desktop operation and mobile.  It will be interesting to see the price and when it is likely to be launched. Also, whether or not they intend to do a 10W version (like the IC703) which was very popular with QRP enthusiasts.

A bit further with the WISPY RX

Although not in the best of shape today, I spotted CX1BP (11131km) and EA8FF on 10m WSPR today.  Not bad for a handful of cheap parts.

I had a bit of trouble today with the SE602 I was using for the TX part of WISPY (it may have been faulty but I was unable to get a clean DSB signal out of mine), so when I get back on the project after a week of interruptions coming up I'm going to go for the simple 2 diode single balanced mixer instead on TX. I know this works well and is less expensive.  The TX part can be simpler than the RX I believe.

Spots with WISPY RX

FR1GZ and others coming through just fine

This is the PC screen capture using WISPY as the 10m receiver for WSPR in the last hour or so. It is working just fine.

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?