IMD of VHF PA - Page 1
Forum section: electronics / rf-microwave

author: e.sotillet
author_rank: Contributor
timestamp: March 25, 2022, 05:21:23 pm
content: Hi folksI'm building a VHF PA consisting of 2 MMIC and one LDMOS (AFT05MS004NT1) for a total gain of 45dB, including an ALC circuit set to 30dBm output level.After the overall test (gain, efficency, etc) I proceed to make a IMD test using 2 0dBm tones 1MHz apart and found thath the intermodulation products were 20~22dBc. My questions are:1.- What IMD value is aceptable or good in a typical VHF comunitacions PA?2.- How I can increase the OIP3 (In addition to increasing the bias current of the output amplifier)?3.- I checked out comercial VHF PA's and usually the uses an overrated output (150W LDMOS for 10W output), achieving at leat -42dBc spurs. Is this the right way to do it?Finally, I remove the 2 input preamps and run the same test, and the tones were -35dBc.Best regards!!

author: TheMG
author_rank: Frequent Contributor
timestamp: March 25, 2022, 06:50:20 pm
content: How much IMD is acceptable depends on a variety of factors. Firstly, are the spurious IMD products within the allowable levels set by the applicable regulations in your country, for your particular frequency band and intended use? Secondly, assuming the IMD does not exceed spurious emissions limits, are they going to cause problems with the quality of the transmitted signal? This will depend a lot on the type of modulation you are using. For instance, with FM it doesn't really matter and actually non-linear amplification class C / class E etc (which is pretty much absolute worst case scenario for IMD). Others like SSB, analog TV broadcast, having low IMD is very important. Many types of digital data modulation do also need relatively low IMD for best performance.In short: there is no single "rule of thumb" as to what is acceptable IMD.Quote from: e.sotillet on March 25, 2022, 05:21:23 pmYes this is one of the commonly used techniques, I've seen it used in a lot of low power (<100W) analog TV broadcast amplifiers, where having very good linearity is important for the best signal quality, and more important than efficiency and heat output. They tend to run most of the amplification stages including the final output at a small percentage of their maximum capability, and the devices are biased very hard, almost into class A operation. The idea is to have the devices operating in the region where they are most linear. The linearity of most devices begins to suffer as you get closer to compression, or on the low side with not enough bias.Clever feedback schemes and signal pre-distortion/pre-correction are other examples of techniques used.Again, the suitability of a particular technique at reducing IMD and improving linearity depends on the type of signal being amplified, frequency, etc.Quote from: e.sotillet on March 25, 2022, 05:21:23 pmAny signal stage can contribute to IMD, and it can indeed add up. So it is important to minimize IMD at every stage.

author: e.sotillet
author_rank: Contributor
timestamp: March 28, 2022, 11:16:18 am
content: The intended use is not limited by regulations because it will be used in a closed space, with FM and some digital modulations. But I want to have some "reference" to know when stop trying to obtaing higher OIP3, -30 to -40dBc seems a good number?Regarding to oversize te output device, I will try with an 30W prototipe to check the OIP3 at 1W output, i will post the results.Best regards

author: eb4fbz
author_rank: Regular Contributor
timestamp: April 10, 2022, 09:19:18 am
content: Looks like you are driving the MMIC driver stages too hard. Ideally the IMD should be dominated by the last stage.

author: virtualparticles
author_rank: Regular Contributor
timestamp: April 21, 2022, 06:12:49 pm
content: LDMOS usually has a "sweet spot" in the bias where the IMD performance is quite good. I built PAs with +60 dBM OIP3 by using 45W parts highly backed off to produce 1W output. The bias was carefully adjusted for best IP3 and then temperature compensated as well. A very slightly different bias was needed at elevated temperature. The resulting bias point was still well into class AB so I wasn't heating the entire room with the thing. I did have power to burn though since it wasn't in a portable product.