Re: Looking for Hotair TX35FM info
Posted: Wed Apr 03, 2024 11:10 pm
It's quite simple:
If you consider The percentage of the output frequency that 75kHz deviation represents against (say) 88MHz, and then the percentage of 107 MHz it is, you'll see that it's less at 107 MHz. Also, the modulation sensitivity of the varicap isn't entirely linear, so if you're trying to use one varicap for both tuning and for the modulation, you'll find that the audio level required for 75kHz deviation will change quite a bit....
There's a further problem with the single varicap approach - BW tried to get their exciter to voltage tune right across the band with a voltage swing of about 10V. This meant that 75kHz deviation needed signal levels in the low millivolts - no wonder that they always had hum problems!
There are further problems with the "at frequency, single varicap" approach, including RF feedback from the PA to the oscillator (causing hum and instability). That's why I always preferred a half (or lower) frequency oscillator, multiplied to the required output frequency. I also always used a separate modulation varicap, coupled to the oscillator with its own low value capacitor. This allowed me to move the modulation varicap bias voltage to the most linear part of its characteristic (minimising deviation distortion), and have a sensible modulation sensitivity (typically a few Volts p-p for 37.5kHz swing - the deviation would be multiplied by the frequency multiplication in the exciter).
One experimental exciter I have here has what amounts to an FM receiver "listening" to the modulation. I take the demodulated audio into a measurement circuit, and I can accurately measure the actual deviation. I can use the derived deviation value to control a peak limiter, to completely eliminate the possibility of over-modulation, and can also drive a real deviation meter on the front panel!
If you consider The percentage of the output frequency that 75kHz deviation represents against (say) 88MHz, and then the percentage of 107 MHz it is, you'll see that it's less at 107 MHz. Also, the modulation sensitivity of the varicap isn't entirely linear, so if you're trying to use one varicap for both tuning and for the modulation, you'll find that the audio level required for 75kHz deviation will change quite a bit....
There's a further problem with the single varicap approach - BW tried to get their exciter to voltage tune right across the band with a voltage swing of about 10V. This meant that 75kHz deviation needed signal levels in the low millivolts - no wonder that they always had hum problems!
There are further problems with the "at frequency, single varicap" approach, including RF feedback from the PA to the oscillator (causing hum and instability). That's why I always preferred a half (or lower) frequency oscillator, multiplied to the required output frequency. I also always used a separate modulation varicap, coupled to the oscillator with its own low value capacitor. This allowed me to move the modulation varicap bias voltage to the most linear part of its characteristic (minimising deviation distortion), and have a sensible modulation sensitivity (typically a few Volts p-p for 37.5kHz swing - the deviation would be multiplied by the frequency multiplication in the exciter).
One experimental exciter I have here has what amounts to an FM receiver "listening" to the modulation. I take the demodulated audio into a measurement circuit, and I can accurately measure the actual deviation. I can use the derived deviation value to control a peak limiter, to completely eliminate the possibility of over-modulation, and can also drive a real deviation meter on the front panel!
thank you so much for the link.