开云体育

Hi Mark,

The thing you might be missing is that the HP 934A is not just a regular mixer, it is a harmonic mixer.? Internal to the HP 934A, the reference input signal feeds a step recovery diode (SRD) circuit, which generates a comb of harmonic frequencies: 1 x Fref, 2 x Fref, 3 x Fref, 4 x Fref, and so on.? This comb of frequencies then mixes with the other input (i.e., the auxiliary output signal from your HP 83590A) in a regular mixer section, which is also inside the HP 934A.? The resulting harmonic mixer output, that arrives at your HP 8709A Synchronizer, is full of the sum and difference frequencies, of the pure sine wave from your 83590A and all the multiples of the reference frequency.? From all of these frequencies, an input filter on the HP 8709A Synchronizer only allows a 20 MHz signal to pass through -- It blocks all the other frequencies that will be present.



For example:
----------------
If your HP 8640A reference generator is tuned to 995 MHz, the 934A internal SRD device will generate a spectral comb of harmonic frequencies: 995 MHz, 1,990 MHz, 2,985 MHz, 3,980 MHz, 4,975 MHz, 5,970 MHz, 6,965 MHz, 7,960 MHz, and more.? As the frequencies get even higher, they will tend to roll-off in amplitude, until they finally become unusable.? So, even though the 8640A only goes up to 1024 MHz, you now have discrete frequencies through the microwave band, which are available to drive the mixer.

Now, assume that the 83590A is tuned to 4,000 MHz.? Your 995 MHz reference signal and all of its harmonics will be added and subtracted from this 4,000 MHz signal.? For simplicity, we will just look at the fundamental and first few harmonics.

reference fundamental frequency (N2 = 1, in the manual's equation)
4,000 MHz - 995 = 3,005 MHz.
4,000 MHz + 995 = 4,995 MHz.

reference 2nd harmonic (N2 = 2, in the manual's equation)
4,000 MHz - 1,990 = 2,010 MHz.
4,000 MHz + 1,990 = 5,990 MHz.

reference 3rd harmonic (N2 = 3, in the manual's equation)
4,000 MHz - 2,985 = 1,015 MHz.
4,000 MHz + 2,985 = 6,985 MHz.

reference 4th harmonic (N2 = 4, in the manual's equation)
4,000 MHz - 3,980 = 20 MHz.
4,000 MHz + 3,980 = 7,980 MHz.

The HP 934A mixer output, that is created by the subtraction of the 4,000 MHz signal and the 4th harmonic of the reference signal, is 20 MHz.? It is this 20 MHz signal that will drive the HP 8709A Synchronizer.? The synchronizer will filter out the other frequencies and not allow them to pass to its internal circuitry.

In the above example, you have just used your HP 8640A to synchronize a 4,000 MHz signal.? This is despite the fact that the 8640A only goes up to 1024 MHz.

As a further exercise: If we plug in the numbers to the (corrected) equation from the 83590A manual, we have:

(HP83590A aux output frequency - 20 MHz) / N2 = the required HP8640A reference frequency
(4,000 MHz - 20 MHz) / 4 = the required HP8640A reference frequency
3,980 MHz / 4 = the required HP8640A reference frequency
995 MHz = the required HP8640A reference frequency

The auxiliary RF output of the HP83590A only covers 2 through 7 GHz.? But, when this auxiliary RF output signal is phase-locked, the main RF output of the HP83590A will also be phase-locked.? The main output covers the full range of 2 through 20 GHz, which is just a multiple of the 2 through 7 GHz auxiliary output range.? The phase-lock example in the HP83590A manual calls this multiple: "N1", where "N1" can be equal to 1,2 or 3.