Radio Remco (using a Racal-Dana 9084)

Remco Bloemen

2009-10-19, last updated 2014-02-27

Today at the university they were cleaning out some of the old equipment that had been laying on shelves for years. Someone tipped me that the Racal-Dana 9084 Synthesised Signal Generator was a particularly interesting piece of equipment, so I tied it to my bicycle and rode home.

To my surprise the generator worked immediately on connecting it to mains and switching it on. I quickly hooked up a scope to play with it. I even measured if the reference signal generator was accurate, since the last time it was calibrated (according to the sticker) was before I was born. It was exactly 1.000 MHz, so well within the accuracy of my scope.

Racal-Dana 9084 on 94 MHz FM
Racal-Dana 9084 on 94 MHz FM

Rigging a FM radio transmitter

Since the generator has an AM and FM modulator and could cover the AM radio band and much of the FM radio band it seemed natural to try this out. Sticking a wire of about half a meter in the centre of the BNC outlet as a makeshift antenna. I first tried AM with an internal 1kHz test-tone signal, which I could somewhat receive on a radio and then switched to FM at 94 MHz, which worked beautifully.

Jimmy rigged FM radio signal producer
Jimmy rigged FM radio signal producer

The next step was to create an audio signal and start broadcasting! I took some more cables and crocodile connectors and connected the headphone out to the input of the modulator. Put Banarama – Venus in a loop (love the baseline at the start of the song) and started broadcasting. At one point I played a 19 kHz sine through the signal, which made the FM stereo indicator light up :). Sound quality was very decent, even though I did not boost the high frequencies as is apparently the standard practice.

I had perfect reception at the neighbours when transmitting at 30 mV. But even on 1 V there was no reception on a friends radio-enabled cellphone in his house 900 m from here. Seems quite obvious in retrospect.

Measuring peak output power

Time to measure the peak output of the transmitter. Since the output is labelled 50 Ω I shorted it with a 56 Ω resistor and measured the root means squared voltage while increasing the signal strength. The dial went all the way to 1 V rms. And then I discovered a button labelled “×2”, pushed it, and the signal shot to 2.16 V rms :D. The signal and generator appeared completely unimpressed by continuous load, so I estimate it’ll run the same on 50 Ω. This results in a total power of:

P=U2R=2.16250=93mW P = \frac{U^2}{R} = \frac{2.16^2}{50} = 93 mW

Quite nice for a device never meant to be a transmitter of any kind.

10 MHz at full blast
10 MHz at full blast
Improvised power measuring device. Improvised power measuring device: The transmitter is loaded with an 56 Ω resistor and the root mean squared voltage is measured with a scope.
Improvised power measuring device. Improvised power measuring device: The transmitter is loaded with an 56 Ω resistor and the root mean squared voltage is measured with a scope.

Since my neighbours are about 10 m away and my friend lives 900 m away I should expect to require 10 log109002102=39dB\cdot \log_{10} {\frac{900^2}{10^2}} = 39 dB more transmitting power. I was transmitting at 30 mV and could go up to 2.16 V so I had 10log102.1620.032=37dB10 \cdot \log_{10}{\frac{2.16^2}{0.03^2}} = 37 dB of headroom, just 2dB2 dB short of reaching my friend!

Surely I must be able to reach my friend if I build a simple bi-pole antenna.

Hi - Nice to read your article, especially as there is a better than 50-50 chance that I tested the unit originally. I worked at Racal from 1977 and was the test department engineer assigned to the project. The unit was a really good performer at the time, but we felt that it was a mistake to stop short of the top of the FM broadcast band as an engineer would be more likely to buy a unit if they could test their tuner with it!

Hope it keeps going for you,

Bob

— Bob Miller 2010-04-24

Hallo Remco, Leuk te lezen over je radio experimenten, al denk ik dat je ze beter op een andere frequentie en met een machtiging daarvoor zou kunnen uitvoeren…… :-) Ik heb sinds kort ook een Racal 9084 en ben op zoek naar de documentatie hiervan. Kun jij me verder helpen?

— Henry Vredegoor 2011-02-02

@Henry: Ik heb er helaas ook geen documentatie bij, wel veel naar gegoogled. De fabrikant bestaat al een tijdje niet meer, dus support is moeilijk te krijgen. Maar met wat kennis en proberen is de voorkant van het apparaat redelijk te begrijpen.

Een zendmachtiging maak ik mij niet zo druk om, dit apparaat heeft minder vermogen dan een FM-babyfoon en ik zet het apparaat maar heel kort aan.

— Remco Bloemen 2011-02-03

I have one of these sig gen’s it was working wonderfully for a long time, then one day the freq readout started flashing and the freq is way off, I guess this means the synth has lost lock. Any idea’s as to what the problem may be and how to fix it ?

Chris

— Chris Cain 2011-05-07