472kHz transverter "progress"?

This afternoon, after a break of a few weeks, I went back to my 10W 472kHz transverter design. The intention was to simply build a second sample to prove repeatability. As is often the case, things didn't quite work out that way: instead I discovered an MF oscillation when drive was removed in some situations. Despite trying all the usual tricks so far, I've not managed to fix it yet. It is not the PA: driving it and the squarer stage directly with a 472kHz signal all is well. It looks like a problem around the oscillator and mixer but so far I have not managed to tame it. Also, and this is probably the same issue, on RX there is a wideband noise resulting in desensitisation. For this evening I have stopped work and will take a fresh look tomorrow. Maybe the double balanced mixer in my original 2010 circuit was not such a bad idea after all.

This is a classic situation when a spectrum analyser would really help as I could see in an instant where the trouble was by poking around with an RF probe and observing the spectrum on the analyser.

Bitsbox - excellent service (again)

A few days ago I decided to order some electronics components to replenish my stocks - resistors, common capacitors, trimmers, FETs etc, so I placed an order with Bitsbox, my favourite UK supplier. The order was acknowledged immediately and the goods arrived next day by first class post. Bitsbox prices are very sensible (resistors 1p each in 50 offs for example) and they charge a fixed £1.75 delivery charge per order for UK first class post.  I've now used this company several times and cannot fault the service. They stock a wide range of parts - certainly most of what I need for VLF-VHF experimental building - and the delivery is super fast. 

G3XBM recommends Bitsbox.

WISPY reports and a red kite

For the last month we seem to have been away from home half the time. The last few days we have been in Yorkshire for a sibling reunion with my wife's brothers.

On the way back home we had a REAL surprise driving along the Leeds ring road. I could not believe my eyes when I saw a red kite flying quite low overhead. These fine forked tailed raptor birds were once confined to a small part of south central Wales but following selective reintroduction they are making a comeback all over the place. We see them near Newmarket quite often, but I was totally surprised to see one driving through urban Leeds!

When I got back home I switched on the WIPSY beacon and was rewarded with a string of reports including a +6dB S/N from Germany - not bad for 100mW. The now regular  report from EA8FF was not long coming as was a report from F5VLY and several other DL stations and G8KNN not far away.

WISPY TX gets better (and even cheaper!)

Today I did a rebuild of the TX only version of the WISPY 10m WSPR beacon to improve it in several ways.  Switching to germanium diodes helps produce a much better looking DSB signal out of the mixer. With silicon diodes there was not quite enough drive. The second change was to remove the 2N3866 PA and replace it with a couple of paralleled up 2N3904s (each with a small emitter resistor so they share current properly). A 2N3866 is around £1.50 whereas a couple of 2N3904s are about 6 pence.  Output is now a clean DSB signal, even without the low pass filter, with a conservative 200mW DSB out (100mW equivalent SSB signal in the WSPR band). This is now a very pleasing little design that can be built for even less.  Within 5 minutes of turning it on it got a remarkable -3dB S/N report from EA8FF over 3000kms away.

The first UK broadcast station

The first public broadcasts in the UK were not by the BBC but a few years before in 1920 from the station 2MT in Writtle, Essex. It was known as Two Emma Toc.

The book "2MT Writtle – The Birth of British Broadcasting" by Tim Wander gives the history of this first station. It is available from Amazon for £18.95.

TX only WSPR beacon

UPDATE 11.9.12: see later post for improved version

WISPY is a full 2-way WSPR transceiver but there may be occasions when just a simple WSPR TX is needed. This is the (current) schematic of the WSPR TX beacon for 10m, but the very same schematic with a different crystal and values should work on any HF band. On lower bands the doubler could be removed and a simple on-frequency crystal substituted.  The doubler approach was the cheapest way for 10m WSPR.

The schematic needs rationalising and some further simplification, so treat this (as always) as a design starting point.

Solar predictions

The NASA solar physics website has some recent updates to the solar cycle peak predictions:

"The current prediction for Sunspot Cycle 24 gives a smoothed sunspot number maximum of about 76 in the Fall of 2013. The smoothed sunspot number (for 2012/02) is already nearly 67 due to the strong peak in late 2011 so the official maximum will be at least this high. We are currently well over three years into Cycle 24. The current predicted and observed size makes this the smallest sunspot cycle since Cycle 14 which had a maximum of 64.2 in February of 1906."

Looks like they are expecting a double peak cycle with a slightly larger peak about a year from now. We'll see. The peak is likely to be very low compared with mega-peaks seen in the last half century.

WISPY-10 schematic (draft A)

OK, I promised to show the schematic of WISPY-10 (there may be versions for lower bands later) today so here it is. I have breadboarded and tested the TX and the RX parts on-air and both work well. In the transceiver I have used the oscillator with an additional buffer (x2) stage on TX and this has still to be tested but I don't expect any issues at all. Also shown is the TX-RX change-over using the back-to-back diodes in the middle of a series tuned circuit on the RX input. On TX the diodes conduct and the capacitor forms part of the output low pass filter.

Updated schematic (an error spotted by G3TFX)

At the moment I am not planning any further work as I want to crack on with the 472kHz work but I expect to return to this in a few weeks' time.

Click on the schematic for a larger, more readable, version. Please let me know if you spot any obvious errors (e.g. where I have put grossly wrong values of components in) so I can update the schematic.

Please feel free to use this as a starting point for your own version. All in, I doubt the components would cost more than £5 (7 euros or 8 dollars) even if all the parts were obtained new.

The 2N3866 runs quite hot in class A and I want to change this biasing in the final version. I may also susbstitute the 2N3866 for a couple of paralleled 2N3904s to save further cost.

The same circuit will work well on PSK31 (pull the crystal to the right frequency of course) but both with WSPR and PSK31 do not over drive the PA. This is a DSB transmitter and there will be splatter either side of the carrier for tens of kHz if driven too hard. Adjust the drive so the output from the PA is a clean, DSB signal free from obvious audio distortion.

WISPY now transmitting!

Breadboard of WISPY TX - 50mW pep WSPR (100mW DSB)

Very pleased with my little 3 transistor DSB 28MHz WSPR transmitter. Since turning it on at 11.34am I've received around 30 reports from stations in Europe despite the power being just 50mW pep ...and as I write my first spot from Africa: -14dB S/N (a good report) from EA8FF in the Canary Islands!

Schematic to follow tomorrow. OK, I know - there is no low pass filter on the breadboard. As the circuit is linear throughout and there is filtering on both stages the output is pretty clean, but I will add a 3-section LPF on the final transceiver version.

This is a very simple, all discrete parts, design using 2 x 2N3904s, 1 x 2N3866 (may be overkill) and a couple of 1N4148 diodes in the mixer.  Again, I use a cheap 14.060MHz crystal, pulled a bit and  doubled, as the LO.

FLEX-6700R hype?

Just read this on a UK supplier's website:

The FLEX-6700R™ Signature Series receiver is based on a new hardware architecture and SmartSDR™ software platform.  This hardware uses Digital Down Conversion (DDC) to convert from RF to digital.  The FLEX-6700R uses two Spectral Capture Units™ (SCU) for the RF to digital conversion and an ultra high performance on-board signal processing and control system to perform demodulation, filtering, and audio stream management.  The result is incredible receiver dynamic range and received signal clarity.

The FLEX-6700R model receiver utilizes two SCUs for reception from 0.33 to 77 MHz plus 135 to 165 MHz and the ability to create up to 8 full featured independent SLICE RECEIVERS.  Optimized preselector filters can be selected for the ham bands for greater out of band rejection.

SmartSDR is the “ecosystem” of the radio platform.  It organizes all the signal processing power in the radios into an advanced radio fabric.  SmartSDR understands the capabilities of each SCU and how to harness its power.  It also comprises of the “presentation layer” or GUI client that the user interacts with.  The FLEX-6700 utilizes an Ethernet connection for high-speed “driverless” access to the graphical user interface (GUI).

Now I know there are people around with too much money and there are some applications when a very good receiver may be of benefit, but would YOU spend £4799.94 on one? This is just the receiver remember! No thanks. I'll continue to build simple rigs for a few pounds and accept their limitations.