[volt-nuts] HP3458A SCAL hardware

[Poul-Henning Kamp]

I am looking at the HP3458A SCAL procedure and for which I am
somewhat short on the required hardware, and have been pondering a
simpler way to perform the SCAL, and would like to hear your comments.

The basic requirement is to supply three AC voltages at two
or three frequencies:

	3-10Vrms    @ 100kHz, 2MHz & 8MHz
	0.3-1Vrms   @ 100kHz       & 8MHz
	30-100mVrms @ 100kHz       & 8MHz

Forget the frequency precision for a moment, that is trivial.

As long as the voltages are within 0.2% (0.017dB?) for the the
various frequencies, the absolute value of the voltage is not
important.

Very few, if any, tone generators can meet that flatness spec, which
is why the cal-procedure specifies use of three hard to get thermal
converters.

I came up with this circuit:


	  -----------+-------+--- 330R ----+-----+-------
                     |       |             |     |
Generator        optional    |           130R   10R
(HP33120A)	 thermal    51R            |     |      HP3458A
		 converter   |            SW1   SW2
                     |       |             |     |
	  -----------+-------+-------------+-----+-------

SW1 and SW2 are either switches or, likely better: RF relays.

I built a birdnest version of this, using SMD resistors and
two standard relays from the junk-heap.

I tested it with my HP3577A, and the results are not discouraging:
The flatness from 100kHz to 2MHz is close to spec, but I loose
about 0.1dB from 2MHz to 8MHz.

The ideal mechanical construction would be a PCB with two 4mm
bananaplugs in one end, ready to plug into the HP3458A and a
BNC in the other end to connect the generator and a couple of
wires to drive the relays.

But I am far from sure I know how to design this PCB, nor
what kind of components are best (SMD/through-hole).

In fact, I am not even sure it is a good idea to have a ground
plane, since the right hand side has variable circuit impedance
depending on the switch positions.

The alternative is to change the circuit to have 50Z all the way
through, and a 50 Ohm terminator on the meter side.

In that case the voltage selection could be done by switching in a
10 and a 20dB attenuator.

Has anybody tried this ?

Any other ideas ?

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