Testing an HP8447D RF preamplifier

[6SN7WGTB]

Just acquired one of the above and it is specified as 26dB gain (±1.5dB) 100kHz to 1.3GHz.

I'm just testing it and would like some assurance that I'm doing this correctly:

Using an IFR 2025 as signal source (50Ω output) and in EMF mode.

BNC to HP (which has 50Ω input and output), then BNC to a Racal Dana 9300 RF mVM.

The gain is between 27 and 27.5dB between 500kHz to 10MHz (the upper practical limit for the 9300), accepting the measurement accuracy of my equipment.

FWIW, the 2025 output measures exactly what it claims on the 9300.

What I am not sure about is should I have a 50Ω termination at the 9300 (which has a 1MΩ input)? Clearly that halves the output and then I'm obviously 6dB down...

[Jon_G4MDC]

That's a definite yes.

[6SN7WGTB]

Originally Posted by Jon_G4MDC

That's a definite yes.

Yes, I figured so to meet the HP output impedance.

That means I'm down 6dB, which seems unlikely.

But, should I have the 2025 in PD, not EMF, mode too? Then I'm back at 27.5dB...

[PETERg0rsq]

As far as I recall, EMF is the voltage when no current is flowing (i.e infinite impedence load).

PD is voltage at terminals with current flowing.
Your amplifier has 50 ohm input impedance, so current will be flowing, so when in PD mode, you will measure the exact voltage output when connected to a 50 ohm load.

What I dont recall is if the PD output needs terminating also, but as you are adjusting the voltage while connected to the 50 ohm input of the amplifier, it probably does not matter!

Hopefully someone will confirm this, or otherwise.

[Cruisin Marine]

I normally set my TF2018 to dBm, This always tallies with associated equipment like the 6910 power/level meter for example.
You can switch the level to emf to see what that is of course.

What I am not sure about is should I have a 50Ω termination at the 9300

I don't personally know those, but be aware, some probes have a 50 ohm termination (on Marconi gear like the TF2603) for example... and some don't!

[Radio Wrangler]

The 8447D is designed to present 50 Ohms to whatever is driving it and it is designed to drive into a 50 Ohms load. The gain spec is under these circumstances.

You can do the gain by comparing levels in dBm or in potential difference. Some amplifiers can behave oddly into open loads so EMF doesn't necessarily go up 6dB. It all depends on how that 50 Ohms Zout was created.

Strictly speaking that 26dB gain is 'Insertion gain' Have a 50 Ohm (output) sig gen driving a 50 Ohm (input) meter and note the meter reading, break the connection and insert the amplifier and check the reading increases 26dB. All connections are therefore matched under both circumstances.

David

[6SN7WGTB]

Thank you all. Understood. The right answer is clearly PD output, the HP terminates and sources 'itself' at 50Ω, and then I terminate the output at the 9300 with 50Ω.

And all is good.

A nice bit of kit given what I paid for it.

Mainly purchased to use with an active RF probe that has ca. 23dB of insertion loss and so with low voltage sources wouldn't drive some kit.

[Radio Wrangler]

There were also two close-field probes which were the bee's doodads for finding EMC escape routes. They needed this amp to get levels up for best floor wiith spectrum analysers.

David