In Retrospect An HF Signal Generator

Dear Don,

Firstly, a thank you to those that have shown an interest and placed orders for the recent project. Page 64, Amplitude Modulation

To explain linear mixing a little further: An example of Linear Mixing is the use of a microphone mixing desk where two or more mics are mixed and the output is the result of the mics and no other signals (excepting harmonics) whereas non-linear mixing produces sum and differences as observed in a superhet mixer.

Explaining the linear mixing in terms of voltage and current is only to show the current increases in proportion to the voltage and the term ‘square law’ is more applicable when explaining power circuits.

To be pedantic, the use of the term ‘Product’ should be called ‘Sum of’ and not the product. Also Intermediate refers to sum and difference and not just the difference. Page 65. First paragraph.

With reference to using varicap diodes back to back:

From Wikipedia

Another common configuration uses two back-to-back (anode to anode) varicap diodes. (See lower left circuit in diagram.) The second varicap effectively replaces the blocking capacitor in the first circuit. This reduces the overall capacitance and the capacitance range by half, but has the advantage of reducing the AC component of voltage across each device and has symmetrical distortion should the AC component possess enough amplitude to bias the varicaps into forward conduction. Page 65, ‘Output’

The voltages (1V) and (2V) are used as an example of 0dB and 6dB in this explanation but we all know that dBs are, of course, ‘ratios’ and do not possess values.

The datasheet shows the MAX452 as having an output impedance of 2Ω but can drive 75Ω directly. I have used this chip in many 50Ω video mixers and amplifiers several years ago. It worked well and it is used here to provide a 50Ω output.

The output from the 220Ω output level pot produces an output at 50Ω and at just about all settings because of the 50Ω sending impedance at the output of the MAX452. To test this, set the level at, say, 1V output using a scope as the display. Place a 50Ω terminating resistor at the scope input and the signal level will drop to 0.5V. Repeat the operation setting the level at 0.5V (without the termination resistor) and then fit the 50Ω terminating resistor and the level seen on the scope will drop to 0.25V. This displays the output impedance of the generator to be 50Ω (or very approximate).

Eric Edwards GW8LJJ

Barry