Teaching Evan the basics of radar signal processing with this baby-block 7-bit Barker code and its matched filter.
Posts Tagged ‘linearity’
Over the past week in the evenings, I’ve managed to cobble a little PA together for the 50-MHz transverter. It’s a near-clone of the PA in the Elecraft XV50 using the Mitsubishi RA30H0608M. Last night, I got it all hooked up and installed in the cabinet. It broke into oscillation when keyed on CW.
The oscillation was about 50.040 MHz. I reasoned that it might be the PA output coupling into the nearby TX RF bandpass filters which are followed by 12 dB of gain before returning to the PA. So, tonight, I added a litle shield between the BPF and the PA board. That seemed to clear things up and I got about 10 watts out. There is a 6-dB pad between the last driver stage and the PA, so I should be able to get it up to about 18-20 watts. But, the first goal will be to check the linearity on SSB.
In other transmit capability news, some boxes and heatsinks arrived for the W6PQL PAs for 903 and 1296 this afternoon.
I recently resurrected the 50-MHz transverter project and have made good headway getting it working. On Friday night, I began the process of tapping holes in the PA module heat spreader. But, despite using plenty of “cutting fluid” (3-in-1 oil), I managed to break a (well-used) tap on the first hole. Since the maximum (linear) power out of the driver stage is 200 mW, I embraced my inner QRPer and put the PA project aside to give the transverter a try in the CQ WW VHF contest.
The 6-meter Yagi had come down in favor of Yagis for 222 and 432 when the loaner FT-736R showed up. So, I scampered up onto the roof and moved Yagis around. I had hoped that this moment would be accompanied by changing out RG-8 coax for LMR-600 and LDF4-50A that are taking up space in the shop and shack. But, I was not ready to commit to cutting that and I still don’t have LMR-600UF for the rotator loops. Plus, I should replace the rotator at the same time. That amounted to too much work for the available time. I really just need to bite the bullet and install a rotating mast for the VHF antennas that’s not so precarious.
Got everything hooked up late on Saturday afternoon, but had to tend to some domestic concerns and was QRT until later in the evening when I heard my neighbor W4EE calling CQ on six SSB. Did not know that he had six! Apparently, this is a new thing for him, too. He was surprised that I didn’t vibrate his radio off the desk like I usually do! Told him I was running 200 mW and everything made sense.
Ended up working a few other locals on Sunday including N3UM, who moved me from 6-meter CW to 2-meter SSB for a quick chat. He just completed the N1DPM active bias mod to his Mirage B2518G, so was eager for an audio report. Sounds good! He said my B3016G sounded good on-frequency, but I haven’t gotten the mod actually inserted into mine yet. Probably that Kenwood (TS-700S) audio making up for the amp’s inferiority…
Eventually, I will be posting more details on the circuit here. This is one of those projects that I would not encourage anyone to duplicate as I have constructed it. However, there may be useful features.
Apologies for the bad pun in the title. I was going to call it “Microwave progress” but could help myself when I thought of this one. Here are some notes from tinkering over the past few days.
After learning that the 1152-MHz LO power was -11 dBm, I inserted a MAR-3 MMIC on the 1296RSU transverter board. W1GHZ shows an MAR-6 on his board, but the ever-astute N3UM noticed that the P1dB for the MAR-6 is 1 dBm, which is well below the nominal 7 dBm level of the ADE-5 mixer. Unfortunately, the MAR-3 has about 12 dB of gain vs the MAR-6’s 20 dB. A little bit of skullin’ lead me to my stash of SGA-4586’s (suggested as an economical front end by W9SZ), which can do >20 dB gain with a P1dB point of 16 dBm or so. Perfect.
Unfortunately, with an 144-MHz IF drive of about 2 dBm, the 1296-MHz output was totally trashed with various mixing products. I spent about 20 minutes searching frantically for the 3-dB SMA attenuator I’d purchased at Dayton. But, I never found it—a sign that my organizational scheme has lost control or that I never actually bought the attenuator. Either are possible. So, I did the next logical thing—I added a Pi-network attenuator between the SGA-4586 “LO boost” MMIC and the ADE-5 mixer. If the Dremel tool didn’t make it’s inventor independently wealthy, it should have. The only SMD resistors that I had on-hand that were realistically appropriate (39.6 and 130 ohms) yielded about 6-7 dB of attenuation, so the effective gain of the SGA-4586/attenuator cascade was probably around 13-15 dB, yielding somewhere around 2-3 dBm of LO. Blech. That’s essentially the same as the MAR-3. The 1296-MHz output with 2 dBm of drive at 144 Mhz was about 2.5 dBm.
The data sheet for the ADE-5 suggests that the conversion loss increases as the LO drive level falls bellow 7 dBm (shocking!), but there are not enough data to show how precipitously it deteriorates. However, essentially 0.5 dB of overall power gain does not jibe with the amount of gain in the system, which should be closer to 20 dB. From this standpoint, perhaps the mixing products were due to IF overdrive instead of LO overdrive. In any event, about 3 dB (instead of 6 or 7 dB) of attenuation in the LO would be a good thing.
Fresh off my mixed success with the W1GHZ 1296RSU (which are neatly packaged in Hammond 1590BB-sized cast aluminum boxes), I wanted to give the 902/903 version (which is still loose boards) a try with the spectrum analyzer. So, I hooked up the 756LO board first. It made -5 dBm at 756 MHz. These numbers are more like what W1GHZ was promising. So, I put the MAR-3 mentioned above onto the 902/903 transverter board. With 2 dBm of 147-MHz drive, the output was a very clean 16 dBm on 903. Score! Must be livin’ right at least half the time. My last DigiKey order included a 1590BB for that transverter, but the 756LO board is a bit longer and I haven’t yet found a suitable case for it.
Thought I was done? Me too.
I admit it. I’m a sucker for this sort of thing. Fred, N1DPM, recently posted to VHFcontesting and the “Stanford” VHF lists that he was selling a bunch of spare microwave gear, including a first-generation DEMI 3456 transverter/LO and some amplifiers to get the output up to 4 watts. Cheap. He had some 2304 stuff, but ye old project fund is pretty much depleted since Dayton and I always try to keep a little bit in reserve for just this sort of opportunity. Anyhow, the transverter is set up for 10 watts of drive on 144 MHz. I gave it 250 mW (24 dBm) from my modified IC-290A (post to follow on this—not rocket science, just bypassed the PA) and the output came up at -9 dBm on 3456 MHz. Once I remove the attenuator from the input, I should be able to get it up to about 13 dBm. At least it seems to work on TX. Need an antenna to try RX because the K3UO beacons are not as close as W3APL.
After chatting with Terry, W8ZN, about whether to keep or sell the AM-6155s (which I did sell to finance a tower), he suggested that I apply the N1DPM “Active Bias” modification to my elderly “160-watt” Mirage 2-meter amplifier. I contacted Fred, N1DPM, and he sent me a copy of his paper Linearization of Solid State “Brick” Amplifiers from the 21st Eastern VHF/UHF Conference (1995), along with some additional notes from his notebook. It’s pretty eye-opening how non-linear the amp is without the modification! I finally gathered the parts and hacked it together last night. Hope to test it soon with a “dummy” transistor and then live on the amp. The braided wires go to the thermal compensation transistor which is thermally bonded to one of the RF power transistors.
It probably won’t be ready for the ARRL June VHF this weekend, though. So, I’ll only be using the amp on CW. That’s no matter since I haven’t hung the low-loss cable (LMR and Heliax), nor have I received a D1010 432-MHz amp that will be on its way to me soon. Everything seems to take longer than it ought to!