Transceiver Architecture 2.23
Commercial and Surplus Crystal Filters
6/20/2017 ~ Short diversion -- OLED noise with a 128X32 OLED Blue Display
Believe it or not this test is important to an upcoming project that will soon be revealed.
73's
Pete N6QW
6/18/2017 ~ In the original post I mentioned that I saw three German Filters on a board for sale from Israel but did not know much about them. Thanks to one of the regular blog readers (Thanks Jim), we now have info on these filters and from what I can see they appear to be symmetrical which is important if they are installed in a bilateral circuit such as the Termination Insensitive Amps and the Z in/out is 1 K which is a 20:1 match to 50 Ohms. A 9 turn to 2 turn matching transformer gets you close 9^2 = 81 and 2^2 = 4. 81/4 = 20.25.
The good news is that they are 8 pole filters which will really tighten up the skirts and resolve the "feathering" issue complained about by one of the 40 Meter SDR police and cited in the post. Thank again Jim for the info and the plot of one of the filters taken "bare" without any matching which would affect the ripple content in the pass band. If that ripple is smoothed your are looking at 60 dB
rejection and pretty narrow skirts.
Don't rely on my analysis but for some this might be a good buy and you get an AM filter too! This is definitely not a Dishal homebrew filter and would be hard to replicate in a cold, dimly lit garage with the only tools being a cheap VOM and an 80 Watt RS soldering iron.
I just bought a board with the three filters -- $34 shipped and that gives me three filters.
Pete N6QW
Having sworn off Dishal and other homebrew filters there has to be some alternatives for those who would like to homebrew a rig. The reasons are many for not homebrewing a filter and chief among these is that it is difficult to achieve consistent results from these somewhat arcane processes without the benefit of an extensive amount of test equipment. Anyone who tells you they did it with just a VOM is Fake News. Now if you just want to try your hand at it --by all means build a filter.
But with the advent of the SDR radios and those who lurk around 40 Meters looking for aberrant signals, there is a strong possibility you will get a report that some one has spotted energy above 2800 Hz (especially on 60M) or that your filter is feathering. (OK try to figure that one out.) Yes a homebrew filter will let you enjoy the experience of making a rig where you literally built everything by hand. But I would bet that in most cases the homebrew filter that results unless you are extremely lucky will not match a commercial filter.
So OK where can one buy a reasonably priced SSB filter? One place is the GQRP Club. Join GQRP, become a SPRAT subscriber and buy a high quality filter for about 12 Pound plus shipping. At a $2 to 1 Pound exchange rate --about $30 with shipping puts one of those jewels in your hands. Many of my rigs have this filter. In fact two W7ZOI designed rigs have two such filters in each rig. The IF is 9.0 MHz and works very well for most applications save 17 Meters. Below is that filter in the Zia Transceiver built in 2014 which uses the Hayward/Kopski Termination Insensitive Amplifiers. The photo right below shows another GQRP filter installed in the LM373 Rig.
For the same amount of money INRAD sells a 4 pole Filter Kit --also on 9.0 MHz. I have two of those filters and they work very well. It is their model # 351. The Crystals are color coded and come with both SMD and Leaded Caps so you can pick your method of building the kit. Below is the #351 in a 2017 project implemented with the SMD caps.
Just yesterday I toured eBay looking at commercial/surplus filters. Wow some one from downtown Serbia is selling Elecraft SSB filters for an amazing price. There are also filters from Kenwood, Yaesu and Icom radios. Some are pricey but some are really at a very good price.
There were several Heathkit filters and some listed starting at $10. I have used a Heathkit filter in a transceiver and it works very well. The only problem --it is rather large. The filter IF is a 3.395 MHz and so this lends itself to a dual conversion approach. In the photo below the signals ahead of this board were converted to frequencies in the 8.8 MHz range. The PTO (lifted from a Ten Tec Triton IV) operating a 5 MHz converted the signals to the IF at 3.395 MHz using the TUF-1 on that board. The relay on the board enabled adding AGC to the IF or ALC. The device is a DGM 3N209 in a circuit developed by G4GXO in Sprat 128 --it is bilateral! In the upper right hand corner is a diode ring modulator/demodulator that had both resistive and capacitive balance. That is not something you find in many homebrew rigs. I built this in 2009.
But just as I saw some really good filters there were some questionable ones. There were several boards from Israel showing three German filters on one board. The price was OK --it is just that I didn't immediately recognize the filters. More research needed here.
The 8.8 MHz (or so) Kenwood Filters look like a good fit with a 45 MHz 1st IF. Many of the commercial filters are 2.6 KHz wide and so would give better results on received audio as well has having a Hi Fi sounding signal on transmit with all of the benefits or presence, brightness, color, with great lows and highs.
Frequently with a homebrew filter you will get tons of complaints about having pinched or restricted audio --typically form those operators using SDR radios with 72 Inch LCD screens. So they can really see your signal!!!!
So expand your horizon's and think beyond homebrew filters -- it avoids a lot of work that frequently results in a marginal filter that unless you are extremely lucky will have a difficult time competing with a $30 or less commercial filter. Oh -- the Heathkit Z in/out = 2K Ohms, the GQRP is 500 Ohms and the INRAD #351 is 200 Ohms. Pretty easy to match and a known quantity!
BTW I bought this filter after finding out more of the specifications and this now lends more weight to my argument -- I just bought three filters for $30 and $4 shipping. These filters are better (being 8 pole) than any 4 or 6 pole Dishal filters. After purchasing the board I got a prod from eBay essentially saying buyers who purchased this board also bought Si5351's --so there are individuals out there who are already on this path. (6/18/2017 N6QW)
That is the other bit of good news. In the past having the filter without the matching BFO crystals was a huge problem. No more fellow homebrewer's. With the Si5351 --you simply change the frequency in the sketch and you are off and running with a new filter. Hooray --that is a problem no longer.
BTW I bought this filter after finding out more of the specifications and this now lends more weight to my argument -- I just bought three filters for $30 and $4 shipping. These filters are better (being 8 pole) than any 4 or 6 pole Dishal filters. After purchasing the board I got a prod from eBay essentially saying buyers who purchased this board also bought Si5351's --so there are individuals out there who are already on this path. (6/18/2017 N6QW)
That is the other bit of good news. In the past having the filter without the matching BFO crystals was a huge problem. No more fellow homebrewer's. With the Si5351 --you simply change the frequency in the sketch and you are off and running with a new filter. Hooray --that is a problem no longer.
73's
Pete N6QW