[volt-nuts] 3458A SCAL

[Dick Moore]

Haven't tried this but I strongly recommend going 50 ohms all the way. Flatness will be better and as you note, the attenuation is easier and also flatter overall.

Best,
Dick Moore


On Jun 21, 2010, at 5:00 AM, volt-nuts-request at febo.com wrote:

> Message: 1
> Date: Sun, 20 Jun 2010 22:18:08 +0000
> From: Poul-Henning Kamp <phk at phk.freebsd.dk>
> Subject: [volt-nuts] HP3458A SCAL hardware
> To: volt-nuts at febo.com
> Message-ID: <5484.1277072288 at critter.freebsd.dk>
> 
> 
> 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 ?
> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe
> Never attribute to malice what can adequately be explained by incompetence.
>