[time-nuts] Picking a good HP 10811

[Bruce Griffiths]

Stanley Reynolds wrote:
> The HP10811 has about +- 10Hz coarse adjustment would it be a good idea to make sure they don't have the same frequency by adjusting them in opposite directions from 10 Mhz before the test, like 10Mhz+4hz, 10Mhz, and 10Mhz-4hz for three. Would assume the common oscillator would be 10 to 20 Khz above or below the DUT.
>
> Or does the sound card method work better with small differences ?
>
> The manual recommends grounding the ECC line during tests. Maybe the test rig could set the ECC different on each one in the test. This would result in a much smaller difference and maybe no difference depending on each oscillator zero volt ECC frequency. ECC is listed as 1Hz range for -5 to +5 volts.
>
> Would attenuation of the DUT signal physically separated from the mixer but after and buffer amp help ? 
>   
Stanley

Adjusting the oscillators in opposite directions is helpful at least in
trouble shooting.
However AFAIK it wont have much effect on the susceptibility to
injection locking as that depends on the degree of mismatch between the
resonator natural frequencies.
There no real substitute for a high reverse isolation buffer.

The classical sound card technique works best with small frequency
differences.
There are variants that work better with larger frequency offsets,
however as yet they are not yet fully debugged/tested.
Grounding the EFC line minimises noise contributions from the EFC
voltage source.

Attenuating the DUT will increase the system noise level.

High level mixers like the Minicircuits ZP3H and the TUF3HSM+ level 17
mixers which have high LO to Rf isolation at 10MHz are well suited.
Operating the RF port below saturation level will minimise the mixer
phase shift tempco.
Terminating the mixer IF port in a capacitor will reduce the mixer noise.

A classical buffer amp with transformer input and output that cascades 3
or more common base stages can have high reverse isolation as well as
low input and output VSWR.
At 10MHz 2N3904's are suitable - they also have less tendency to break
into VHF/UHF oscillation than higher frequency transistors.

Shielding everything well also helps.
Build the buffer/isolation amplifiers in individual shielded enclosures.

Bruce