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Old 22nd Jan 2022, 3:38 pm   #1
G0HZU_JMR
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Default Making and Using an RF Levelling Amplifier

Hi, there has been some interest expressed in my homebrew RF levelling amplifier so I've set up a thread to describe the way it works and what it can be used for. There's also info on how to make one and this info will be expanded on as the thread progresses. I hope you find it interesting or useful.

Going back to the 1960s or even further HP used a source levelling system in their scalar network analysers as in the first image below. This uses what may at first appear to be a flawed form of resistive splitter because it only uses two 50R resistors. This would appear to cause an impedance mismatch. However, because of the AGC/normalisation provided by this system a virtual ground is formed at the feedpoint of the splitter. This means the source impedance fed to the DUT is defined by the quality of the series 50R resistor in the RF splitter. The levelling is achieved because the level detected at the reference channel is used to level the source via normalisation/AGC. See the first image below. For accurate measurements with an old and basic system like this it really is important to have a source with a very low VSWR. It's also important to have a system that can constantly correct any level changes and keep the source power at a consistent level.

It's also possible to use this system with a signal/sweep generator with an RF power meter placed in a feedback path as part of an ALC system. See the second image below. This uses a classic HP 83752A sweep generator, the 11667 resistive splitter and a power meter to make up a the auto levelling system. This 20GHz sweep generator has an external ALC port fitted and this port connects to an internal levelling amplifier that keeps the RF output level levelled via ALC action. This setup ensures that the source impedance of the system has extremely low VSWR and it is defined by the quality of the series 50R resistor in the RF splitter. The accuracy of the levelling is defined by the accuracy of the power sensor used. This forms a very elegant system that minimises measurement uncertainty.

Few signal generators offer external ALC like this so I put together a very crude levelling amplifier that can achieve a similar result using external AM modulation. The signal generator must support external AM modulation and the modulation frequency range must operate down to DC for the system to work. See the third image below. This shows a block diagram of a typical working system using my levelling amplifier. It's worth reading all the comments in this block diagram as this can help understand how to set the system up and how to avoid common mistakes that can lead to a non-working system.

It is also possible to replace the exotic RF power meter with a low barrier Schottky diode detector and I'll give some info and advice about this option later in the thread.
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Old 22nd Jan 2022, 3:47 pm   #2
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Default Re: Making and Using an RF Levelling Amplifier

See below for an external view of my levelling amplifier. It looks OK on the outside but it is built using the ugly method internally. The second image shows a schematic for it. I've only just created the schematic because up until now it was in my head. This levelling amplifier rarely gets used because I'm lucky to own a HP 83752A 20 GHz sweeper that has an internal levelling amplifier.

I don't think it's easy to make a universal levelling amplifier as the system is prone to instability especially if the power meter has a slow response time. It's really easy to get gain around the loop at 180degrees phase shift if the power meter is slow to read/respond and the system will just go unstable.

I think this is why the old thermistor based HP432A power meter was popular for use here. It responds much faster than a thermocouple power meter and it also has manual range selection. If you do use a thermocouple power meter then make sure it is set to manual range mode and try and avoid using it at power levels below about -12dBm or the meter will be too slow to respond.

It is possible to slug the response time of the levelling amp to compensate for this but it does make the system painfully slow to use. I'll dig out a few sig gen models and try a few power meter types to demonstrate what works and what doesn't. I know the little Marconi 2022C supports external AM down to DC and it works in this system.

The modern Agilent/Keysight E4419 power meters work in this system and so does the older HP432A/478A power meter and also the old Anritsu ML4803A power meter. As long as the power meter has a recorder out BNC connector on the rear panel there is a good chance it will work OK in a system like this.

The levelling amplifier shown below does work well but it probably needs polishing to make it closer to a universal design. In other words, don't rush out and build a copy and expect it all to work with your sig gen and power meter. The power meter might be too slow and this could cause instability and also your sig gen might not support external AM down to DC. Also, make sure there is a some form of current limiting at the op-amp output and that your sig gen can tolerate positive and negative voltages at the external mod input connector. I've limited the op-amp in the levelling amplifier to +/-5V and fitted a series 120R resistor. The op-amp is a JFET type and can probably only produce +/-4V at the output with limited current capability. This should be safe/OK for most sig gen types.
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Old 22nd Jan 2022, 3:56 pm   #3
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Default Re: Making and Using an RF Levelling Amplifier

The 11667 RF splitter is really exotic and expensive and works up to 26GHz. Some versions work up to 67GHz! I think the B version with 3.5mm connectors (compatible with SMA) costs about £1500 new.

It is possible to make a homebrew version using parallel 100R SMD resistors and this can work well to about 6GHz. I have used my homebrew 11667 splitter to level the HP83752A up as high as 18GHz but the source VSWR will not be good up at 18GHz. I can show how I made this but even a basic attempt at making one using SMD resistors on a small PCB should work very well up to 1GHz or so.
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Old 22nd Jan 2022, 4:49 pm   #4
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Default Re: Making and Using an RF Levelling Amplifier

Probably the most relevant (vintage) use for a system like this would be to test or compare the performance of homebrew RF detectors or probes as it can ensure that the test source is very flat across a wide frequency range and the source VSWR will be very low and this minimises mismatch uncertainty.

This system can work really well and I've used it to compare my various RF power meter heads and also to compare the performance of a classic HP 8473C low barrier Schottky diode detector and also to compare this against homebrew copies of the 8473C. So this technology isn't only relevant to RF/microwave labs or calibration houses. It can be used at LF, HF and VHF as well.

This system can level most signal generators and also improve the source VSWR such that it is typically better than 1.05:1. The most basic RF power sensor to include in the feedback path is a homebrew Schottky diode detector. A quick and dirty design can be constructed that will work really well up through VHF but to get beyond this does take some care and a few tweaks to the detector design to minimise the effect of package inductance and capacitance in the diode itself.
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Old 22nd Jan 2022, 5:18 pm   #5
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Default Re: Making and Using an RF Levelling Amplifier

Note that there is an error in the hastily drawn schematic. The top of VR1 should really go to the regulated 5V node rather than the other end of the 12R resistor R2.
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Old 23rd Jan 2022, 10:50 am   #6
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Default Re: Making and Using an RF Levelling Amplifier

Jeremy, I am not familiar with the previous discussions which lead to this thread, and I'm struggling a little in understanding it.

By 'levelling amplifier', do you mean a circuit intended to give the same RF output level, irrespective on the level being fed in to it?

It's occurred to me in the past that such a circuit would well with a DFC, which need a minimum RF level to drive them, but really do not like being over driven.

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Old 23rd Jan 2022, 11:49 am   #7
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Default Re: Making and Using an RF Levelling Amplifier

It is a little DC amplifier needed when you use an external detector to control the output of a signal generator.

The external detector could be anything from a home-made one with one or two schottky RF diodes, to a precision RF/microwave power meter.

The RF output of the signal generator has to go into a splitter. One output goes to do the job in hand, the other output drives the detector. The output of the detector drives the 'Levelling amp' which drives the AM input of the signal generator. The signal generator has to be set for External DC AM mode.

The advantages of this arrangement are that you can get a more precisely controlled output power level, whether that means in absolute level or in flatness.

Swept signal generators (sweepers) often have facilities to work with an external detector so that levelling can be done based on the level at some different place in the test set-up, to compensate for cables or other test items.

The range of response times of various detectors means that there isn't one size fits all. Sometimes timeconstants have to be added to keep the level-controlling feedback loop stable.

David

Edit, the big disadvantage to external levelling is that the dynamic range of most detectors is an awful lot less than the output range of typical lab sig gens, so if you want to level a low level signal source, you have to provide external attenuators of the needed accuracy and flatness... and screening!
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Old 23rd Jan 2022, 12:45 pm   #8
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Default Re: Making and Using an RF Levelling Amplifier

Thanks for that; I don't have sig gens with such modern features, so not something I will be able to make use of.

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Old 23rd Jan 2022, 2:33 pm   #9
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Default Re: Making and Using an RF Levelling Amplifier

It is possible to lash something up for low frequency use that doesn't require a modern sig gen but the sig gen would have to have low harmonic content if a low cost diode detector was used as the level sensor.

See below for my fairly hideous prototype. This uses a chunky PIN diode attenuator that works well even on the LW band as the diode has a very long carrier lifetime. A dual gate MOSFET attenuator could be used here as well.

The setup below levels really well from LF through to about 100MHz. It uses a diode detector with a negative output voltage. The system has a levelling range of about 20dB and this is usually enough for most things. Obviously, it would work best if fitted in a nice enclosure and if SMD parts were used for the RF splitter and diode detector. However, the board below was intended as a hasty prototype to prove I could get something that would level a sig gen without the system going unstable. There's nothing expensive there apart from the PIN diode and this could be replaced with a different type of attenuator. The attenuator doesn't have to be accurate, it's inaccuracy is corrected by the feedback.

I used two op-amps in series for this so I could break the loop open and plot a Bode plot to look at stability margins. However, I never bothered to try this. It works really well and is really fast because the diode detector response is really fast. With the above setup any type of cw sig gen could be used, even a vintage one as long as the generator doesn't produce high harmonic distortion levels.
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