Having had a look at the block & cct. diag., it looks similar to the design published in the RSGB Handbook, 4th. edition, pages 19.30 etc. I haven't tried that particular cct., but I don't see any part in it that looks an 'impossible-to-get-now' item, with the possible exception of the coils: hand-wound on Aladdin F804 7/16" formers; the more common 1/4" should be a good substitute, or even formers salvaged from old valve IF cans.
The only negative comment I can make about the RSGB design is that the amplitude of the resultant FM-ed wave varies with the chosen centre frequency, which renders any attempt to calibrate the amplitude of the output waveform impossible. However, once the centre freq. has been chosen, the amplitude appears to remain constant over the swept bandwidth. The tuning range is 50 kHz to 2 MHz, with a max. deviation of 60 kHz. Not as extensive as the Jason kit unit, sure, but adequate for most common radio IF alignment tasks.
As with the Jason kit, an external modulating wave is required; unlike the Jason Kit, which uses a sine-wave, this design refers to a traditional ramp. But producing that is not a problem: for example, a unijunction transistor oscillator with the charging cap. fed from a constant-current source will provide a linear sweep, amongst other possibilities. (Anyone got a gas-filled Thyratron spare? :D)
Looks like another 'one day round tuit' project coming my way!
Al.
The only negative comment I can make about the RSGB design is that the amplitude of the resultant FM-ed wave varies with the chosen centre frequency, which renders any attempt to calibrate the amplitude of the output waveform impossible. However, once the centre freq. has been chosen, the amplitude appears to remain constant over the swept bandwidth. The tuning range is 50 kHz to 2 MHz, with a max. deviation of 60 kHz. Not as extensive as the Jason kit unit, sure, but adequate for most common radio IF alignment tasks.
As with the Jason kit, an external modulating wave is required; unlike the Jason Kit, which uses a sine-wave, this design refers to a traditional ramp. But producing that is not a problem: for example, a unijunction transistor oscillator with the charging cap. fed from a constant-current source will provide a linear sweep, amongst other possibilities. (Anyone got a gas-filled Thyratron spare? :D)
Looks like another 'one day round tuit' project coming my way!
Al.
