High harmonic content #380
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See #381 (comment) for more context |
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I connected the Booster to 400MHz, -20dBm signal and probed the traces with SA and 1:10 probe at the output of I, II and III stage and what I got: So, we have a lot of distortion even for relatively low power For 400Mhz and -18dBm, I got It means that it's the second stage who is guilty. |
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That's interesting. And good as it means we may be able to significantly improve this without compromising the power efficiency... |
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I removed the output TVS and the 2-nd harmonics is -21.8dB |
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What does the feedback do to the amp's P1dB? |
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I've just removed it and it seems it does nothing for 400MHz. |
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At -40dBm of input power, the II stage has -46dB of 2nd harmonics |
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Are these numbers dBc? |
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Yes |
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It looks like the PHA-1+ amplifier is working with higher power than planned... |
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OK, so I set -18dBm, connected the probe and the SA shows a peak at -39.9dBm (after 1.5 SMA cable which has 0.5dB loss); so my probe is actually -21.4dB |
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PHA-1+ amplifier is working in very comfortable conditions. |
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Let's see how it behaves loaded with just SA... |
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With -18dBm of input power, it shows -2dBm and 2-nd harmonic is -46dBc |
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So there is something wrong with the III stage input impedance |
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I replaced L2 with a 300R resistor as in the DS, and it's the same. Let's see what VNA shows... |
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the S11 is lower than -10dBm, but of course, that won't show us nonlinearities... |
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what we can do is to add an attenuator between II and III stage and later on remove one before the II stage. That would isolate the stages... |
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I inserted a 7dB attenuator between the stages and the second harmonic dropped to -28dBc. After I increased the input signal by 7dB the 2-nd harmonics is still at -21dBc |
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It looks like we have to go for the higher power amplifier. I identified one with identical package, supplied from 5V and capable of delivering +30dBm P1dB. I ordered a few pieces and will give it a try next week |
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@gkasprow out of curiosity: (a) are your measurements consistent with the P1dB of the current amplifier? (b) what does the feedback do here, are we sure it is unconnected to this? |
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the feedback is supposed to lower the gain slightly and improve linearity. But it is 1k : (50R || 50R) so it's influence is negligible. |
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okay, makes sense. For testing, this sounds like a really sensible plan. If we're going to make changes to the RF section it might be worth having a think about simplifying things. |
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To elaborate a little here... we currently have:
To give a total gain of |
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the input attenuator was added to protect the input stage against overload |
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For my own interest, here are a few MCL options....
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It's a question for all the stakeholders here, but I wonder if we'd be okay with removing all the attenuators and going for a https://www.minicircuits.com/pdfs/TSS-13LN+.pdf as a single-stage amplifier. Cheaper, simpler design with less power consumption. Lower overall gain which I personally don't think is a bad thing. Lower noise figure. A bit less input protection, but should be fine with the diodes. It would mean we have a bad VSWR with the interlock tripped (due to no input attenuator + reflective switch). But I don't see that as being a problem. If it is, we can always use a non-reflective switch (IIRC the motivation for the reflective switch was the switching transients, but that doesn't seem to have been any better with the reflective switch) |
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I ordered NPA1007. It has a similar package and the same voltage but twice higher power. I will give it a try. |
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I installed NPA1007 and the situation is nearly the same apart from lower gain. Default amplifier configuration With NPA1007: I cannot set a higher current than 250mA. |
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So, we can make a special version of Booster, dedicated to high-linearity applications. |
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RFPD3580 is another interesting option, we would avoid connecting two stages in parallel |
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I ordered a few push-pull wideband amplifiers, will give them a try. |
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@gkasprow thanks!
If that's the situation here then I agree there isn't a design issue so much as a deliberate choice made. We wanted something with high power consumption for cases where linearity isn't particularly important. As you say, there is a reason this dissipates much less power than an equivalent MCL amplifier; we pay a price in terms of linearity. This should be clearly communicated to users of Booster. If people want something with better linearity then we can look at designing a replacement RF circuit. First though it would be really helpful to have a clear spec for what is actually wanted from the design (bandwidth, max power, etc). I suspect that taking the current design but aiming for more linearity won't actually provide the optimal solution for people |
Why can't you go higher than 250mA? What limits this? |
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Right. We can make an assembly variant with NPA1007 which has a similar package and pinout (only bias enters separate pin). |
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what's the downside of using NPA1007 as standard and just using a lower current by default? |
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250mA seems to the max due to thermal dissipation. I damaged the FET after running with a higher current. We are also limited by Booster 28V LDO - it trips. |
Okay, so 7W quiescent max.
Okay, good to know. IIRC this was a design choice by us in setting the foldback limiting resistors; the LDO itself can supply a lot more, but I may be wrong... |
exactly. |
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I tried the CATV amplifier. It works at 28V and provides -40.3dBc of second harmonics and -47dBc of third harmonics at 30dBm of output power. |
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It seems feasible to make such an amp compatible with existing mechanics. |
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some photos |
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This particular hybrid has the potential to work at 12V. I mean simple EEM RF amp idea |
How practical is it to build new products based on obsolete CATV amps? They are really nice amps, if you can get them |
If you keep a stock of a few hundred, that makes sense. |
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Another amplifier I ordered from Mouser 2 weeks ago just arrived. Now it takes even 10 days to complete the orders. |
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I integrated the amplifier with the Booster RF board. I removed the attenuators and the first stage because it made the 2-nd harmonics worse. I left only PHA-1+. The overall gain is 30dB which matches Urukul well. And now this is what I get: |
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It's time to design new boards for Booster-HL version |
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And this is the new PCB. The amp needs a simple aluminum adapter to dissipate the heat. It can also be screwed directly to the wall, but that would break compatibility with existing mechanics. |
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Protection diodes on the input and output of an amplifier are generally a bad idea, they are non-linear elements that create harmonics and intermod distortion, even well below their voltage knee. Diodes are designed and marketed for datalines and such, not RF signals. For RF equipment that might be exposed to lightning strikes (outdoor antennas, CATV, etc.), protection usually involves shunt quarter-wave stubs, lumped element filters, and sometimes gas discharge tubes (GDT). Also, once a component is assembled onto a circuit, the ESD tolerance is greatly increased by filtering, decoupling, trace capacitances, etc. For a lab amplifier like this: remove the diodes, slap a sticker onto the front panel that says "+24dBm max input", and maybe add a lowpass filter to the output. |
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@alandoak you are generally right. But in the case of this amp, we don't really care about the linearity. Without these diodes, we observed several serious damages to the first, second, and third stages. You can find all the history of the debugging in issues, which ended up with the introduction of the TVS diodes. Thanks to these diodes, the Booster is bullet-proof. |
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During debugging of nonlinearity, the TVS were the first components I removed :) |
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It's not an issue, it's a feature. Closing. |
One of the users reported that Booster has high harmonic content. Here are the measurements at 150 MHz Signal, (-18 dbm). Signal was generated by Urukul
Comparing with MiniCircuits amp
The Booster was optimized for efficiency, not for linearity.
Let's see what we can do about it
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