How is wideband FM implemented on the E4400B signal generator?

[Twigg]

The Agilent E4400B (formerly HP ESG-something) is a <1GHz signal generator. One of its features is that it can take a <10MHz external signal and turn it into frequency modulation on the output carrier signal (like an incredibly wideband VCO). However, the device can also be externally referenced with a 10MHz clock signal and still perform frequency modulation. How do they achieve both FM and external clocking simultaneously? Everything I came up with wouldn't enable such a high modulation bandwidth.

 

[Baluncore]

However, the device can also be externally referenced with a 10MHz clock signal and still perform frequency modulation. How do they achieve both FM and external clocking simultaneously?

There are ways to synthesise FM by analogue preprocessing, followed by phase modulation of the carrier using an IQ mixer.
What PM methods have you considered?

 

[Twigg]

There are ways to synthesise FM by analogue preprocessing, followed by phase modulation.

Can you elaborate?

What PM methods have you considered?

I am assuming the E4400B is DDS-based (as opposed to analog VCO and PLL), so correct me if I'm wrong about that. The first thing I thought was they could do analog-to-digital conversion, do some very quick digital multiplication by the FM deviation (Hz per volt) (this is a user setting), and then quickly write the new phase to the DDS accumulator. But 10MHz just seems really, really fast to do that much digital work, especially for a device from 2000ish, and for a non-essential feature.

The other possibility I considered is if they actually intentionally offset the phase of the local clock that sets the sample rate for the DDS's DAC. Something like biasing the output voltage of the phase detector in the PLL that references that DAC clock to the external clock. This would require the PLL to have a feedback bandwidth at least 10MHz to keep up with the input signal, and that doesn't seem right either.

Am I missing something much simpler?

 

[Baluncore]

Am I missing something much simpler?

Yes. FM by PM is called "Indirect FM". Google FM by PM.
There are limitations due to PM that are usually overcome by frequency multiplication.
Consider integrating the modulating signal, then use that to phase modulate a 10 MHz reference. Then frequency multiply that before using it to clock a DDS.

Download from; https://connectlp.keysight.com/e/f2
https://www.keysight.com/au/en/assets/9018-01640/service-manuals/9018-01640.pdf?success=true
Start on page 34

 

[berkeman]

is a <1GHz signal generator. One of its features is that it can take a <10MHz external signal and turn it into frequency modulation on the output carrier signal (like an incredibly wideband VCO).

Sorry, why is a 10MHz modulation of a 1GHz carrier extreme modulation?

 

[Twigg]

followed by phase modulation of the carrier using an IQ mixer

Yes. FM by PM is called "Indirect FM". Google FM by PM.

Well darn, I feel really silly right now. Thanks

Consider integrating the modulating signal, then use that to phase modulate a 10 MHz reference. Then frequency multiply that before using it to clock a DDS.

That makes a whole lot of sense. So in that case your modulating bandwidth would be the base bandwidth of the IQ modulator times the frequency multiplier, right? (Edit: No it isn't. See post #8.)

Sorry, why is a 10MHz modulation of a 1GHz carrier extreme modulation?

I only use FM in a laboratory setting, so I don't have a good sense of what's considered "extreme" in a larger context. It just seems extremely fast relative to cases I'm familiar with like FM radio (10's of kHz modulation of a 100's of MHz carrier) or bluetooth's FSK modulation (~1MHz bandwidth out of 2.5GHz). Are there more extreme examples out there?

 

[Baluncore]

Well darn, I feel really silly right now.

You should be really happy, because you are more than sufficiently intelligent to ask your question on the Physics Forums. We can't all have the same experience, and no one knows everything. Your questions and answers are similarly enlightening.

 

[Baluncore]

So in that case your modulating bandwidth would be the base bandwidth of the IQ modulator times the frequency multiplier, right?

The IQ modulator is just one way to phase shift a carrier. In effect, it is performing a vector rotation by complex multiplication of the baseband, by the RF carrier as I and Q signals. IQ modulators are commonly used in mobile phones, so have become smaller, cheaper, lower power and more accurate.

The bandwidth of the FM signal, which is related to the carrier frequency deviation, is frequency multiplied, along with the centre frequency of the carrier.

The spectral content of the FM remains the same, which is why music over FM radio stays in tune.

 

[Twigg]

We can't all have the same experience, and no one knows everything.

This is very true. And thanks for the encouraging words!

The spectral content of the FM remains the same, which is why music over FM radio stays in tune.

Recognizing this helps clear things up a lot. Thanks!