Intro
I had briefly considered using direct conversion receiver for reception. Minimal cost, no AGC system to switch off (or work around disabling). After some discussion with Peter G3PLX and others I was convinced it was not a good idea as the image sideband would add to the noise readings and also allow unwanted signals through.
What it needed was a direct conversion receiver that had two outputs with a 90 degrees phase difference (phase and quadrature) , P and Q outputs. This would mean that the unwanted sideband could be supressed by the GB3RAL program (after an update by Peter).
So I pursued the Plessey superhet receiver project for a while...
Then in February 2007 the following post from Daniel M0ERA appeared on the fivemegs reflector:
would you be interested running your own 5 MHz beacon monitor? You would always know the state of the band. You could also upload the data to the 5 MHz Log, or even add to the realtime server http://g4irx.nowindows.net/fivemegs/comparison.php What I have in mind is a Softrock-60 receiver, similar to the ones advertised in Practical Wireless and RadCom. They are little receivers which have a single-frequency oscillator with I/Q output. The audio then gets fed into a computer where a program monitors the signal strength of the beacons. Softrock receivers are available as kits, but consist of only a few parts. Building time is around 5 hours. Normally, they run with software like PowerSDR, which gives them multi-mode decoding, variable bandwidth filters, you name it. To make the most out of PowerSDR, you would need a powerful computer with an expensive soundcard (see review in latest RadCom). But to use a Softrock receiver for beacon monitoring on 5.290 MHz, a simple soundcard and a simple computer would be perfectly adequate. It only needs to measure signal strength of one Hz bandwidth, after all. Apart from a Softrock-60, a computer, a simple audio cable and a small power supply, you would only need an antenna (eg a small loop antenna) to have your own beacon monitoring station. There is no need for a sophisticated computer interface. Depending on prices for the xtals, and how many of you would be interested, the kits will cost between 11 and 23 GBP.
So of course I was extremely excited!
Ordering the Parts
The Softrock kits - Softrock Lite V6.2 - were available from Tony Parks, the designer and supplier, but would require a dedicated crystal for a 5288.5kHz local oscillator. These were available to order from Quartslab in the UK, with a 3-week delivery time. I decided to order two kits from Tony, less crystals. Ordering with Paypal (and a favourable exchange rate!) the total cost came to $24 (12.70GBP).
Three crystals were ordered from Quartslab, 22.50GBP. Two for building and one spare due to their minumum order charge.
Crystal Specs:
- Frequency 21154 kHz
- Fundamental mode, 50pF capacitance
- Holder HC49/U
Building the RX
The Softrock has some surface mount parts - chip capacitors and ic's. I don't have much in the way of specialist soldering tools for SMD so I used an Antex C 15 watt iron with a 1mm tip. I must have been lucky as both kits worked first time.
The hardest bit was winding the toroids and making sure I'd got the phasing correct on the bifilar windings. If you build one and find the sensitivity is low then this would be a good place to start!
PCB top
PCB bottom
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Calibration
With my QuartzLab crystals the Local Oscillator measures around 5288.25kHz giving me a beat note of 1750Hz.
On my second receiver I've changed C4 from 100pf to 47pf - this moved the LO to 5288.48kHz. If this is not close enough or if you want more adjustment range then you can fit e.g. a 27pf fixed capacitor in parallel with a 30pf trimmer.
I have found the stability of the first receiver to be very good, staying within the ± 25Hz range of the GB3RAL software with the "track freq" disabled.
| Component | Value | Notes |
|---|---|---|
| C3 | 150pF | |
| C4 | 100pF | I changed to 47pF |
| C21 | 150pF | |
| C22 | 2200pF | |
| X1 | 21154 kHz | 50pF parallel resonance. Set JP1 to X4 |
| L1 | 43 turns #30 on T25-2 core (6.3 uH) #30 may overlap - I used finer wire. | |
| L2 | 10 turns #30 on primary and 5 turns bifilar on each of the two secondaries. Wound over top of primary on a T25-2 core. 0.35 uH on primary. | |
Power Supply
One thing I did find critical was the power supply. I use a linear plug-top PSU which is not shared with any other equipment. I did try a switched-mode but this produced spurs every 100Hz on the beacon PC spectrum display! The current requirement is low, the Softrock draws 30mA at 12v, it has an on-board 5v regulator IC.
Sensitivity
Initially I thought it was rather deaf until I discovered by checking the image frequency that I'd got the P and Q outputs reversed so it was responding to the lower sideband. Once reversed, with the beacon program in run mode I can see a marker down to -127dBm (0.1uv). The opposite sideband supression was initially around 35-40dB, but after optimising the settings in the GB3RAL.EXE program I find I can get around 70dB.
The amplitude response of the software is very linear up to the 100dB limit.
Softrock in a box. The switch on the front
is to swap the I and Q channels for sideband reversal. Audio is fed from a 3.5mm stereo
socket on the rear, a BNC connector is used for the aerial feed. The power input goes to
a standard 2.1mm connector.
Here is a waterfall trace of GB3WES displayed on the
ROCKY SDR software. The SDR software is not used for the beacon monitoring but
can be useful for setting up the Softrock intitially (GB3RAL.EXE doesn't provide a
demodulated audio output).
Finally
It has now been online since 29th April 2007. I'm very happy with the performance.
Here are the links to the suppliers:
- Softrock Lite V6.2 is obtained from Tony Parks KB9YIG via the Softrock 40 yahoo group.
- Crystals can be optained from Quartslab. Note their prices have increased slightly since they re-located to EI land.
Good luck, and I hope to hear from you if you decide to build one and join in!
© Nick B. G4IRX. Last update: 26 August 2007 19:39. UTC