EMRFD Message Archive 3761
Message Date From Subject 3761 2009-11-10 07:20:42 bkopski AN IMPROVED RF BENCH GENERATOR Back in the Spring of 2008 there was a discussion on the BITX Group about RF test generators. About that time I had finished one of my own and posted the details into that discussion. The post was #4730.
The generator tunes from about 0.5 MHz to 50 MHz in 5 overlapping ranges. It features a very level RF output over that entire range as well as an adjustable output level of about 0 dBm to +15 dBm. Harmonics are everywhere less than 30 dBc.
More recently I've been reviewing that design with an eye to replacing the tuning cap / mechanism for finer adjustment. In the process, upon reviewing present performance, I came to the realization that the largest harmonics present in the output were the 2nds and I thought this curious. Upon a further look I realized this was likely the result of the leveling circuit implementation and this indeed proved to be true.
A review of the Block Diagram in the EMRFD Files section shows the overall generator circuit with its leveling loop. (Two photos are in the K3NHI Photos section.) The basic generator circuit (in BLACK) is a familiar Hartley FET oscillator with band switching followed by wideband unity buffers. True to classic form, there is a diode limiter in place on top of the tank. However, instead of it being a diode to ground, this diode is returned to a variable DC pedestal voltage established by the leveling loop. This in turn controls the signal swing on the tank and therefore on the final output.
Operation of the leveling loop begins with the desired output level being set with the RF OUTPUT LEVEL pot. Signal peaks at the output are sampled with a simple diode detector circuit, buffered, amplified, and fed back as the tank-associated limiting diode pedestal voltage. This all works very well but in that only the positive-going swings on the tank get "tugged down" by the leveling diode the waveform is slightly amplitude-unsymmetrical. This accounts for much of the second harmonic content in the final output.
Once I understood this it seemed reasonable to make the tank swing limiting symmetrical. This was accomplished with a simple "tracking limiting diode" circuit addition as shown in RED on the schematic. The result: the second is on average more than 14 dB below previous levels throughout the entire tuning range.
The average second harmonic level measured at +10 dBm output over the entire tuning range is more than -57 dBc. The worst value is (not surprisingly) to be found at the highest frequency (~53 MHz) and is -34 dBc the result of the obvious beginnings of slew limiting in the buffer stages. As an incidental, output flatness is better than +/- 0.5 dB over nearly the entire tuning range. It deteriorates to about 1.3 dB reduction at 53 MHz and +15 dBm output again obvious slew limiting.
I have not looked at whether this dual limiter diode technique would be as useful in the classic FET Hartley (or Colpitts) "VFO" circuit but it certainly performs impressively in my HB RF generator!
3772 2009-11-15 05:35:15 g3oth Re: AN IMPROVED RF BENCH GENERATOR Thank you to the originator of this neat design, I saw your original post last year on the BITX group and your subsequent one last week, so came here to EMRFD to look in their files secti