looking for a 455 KHz notch filter module - Page 1
Forum section: electronics / rf-microwave

author: Tony_G
author_rank: Super Contributor
timestamp: September 01, 2016, 04:52:01 am
content: Hey All,I have an 8902A that the front end RF amplifier (U1 for those following along with the service manual) appears to have gone out of spec (low).The cost to repair is apparently ~$8K (from Keysight) so I don't think I'll be taking them up on that offer.However, I was thinking about trying a field expedient repair and just replacing U1 with an 25-30dB 0.1-2GHz RF amplifier module and some short SMA (I think, can't remember) pigtails.However, to be remotely in the ballpark of working to spec I also need to get a 455 KHz notch filter. I can find many filters where they're designed to pass 455KHz but not one that would block it.This has to be because I'm not using the right terminology rather than the fact that no one makes these any more.Any suggestions on what I should search for?Thanks,TonyG

author: uncle_bob
author_rank: Unknown
timestamp: September 01, 2016, 03:39:04 pm
content: Quote from: Tony_G on September 01, 2016, 04:52:01 amHiThe standard 455 KHz notch filter was always done with a 455 KHz crystal and some external components. They never showed up as a packaged part. The reason was that in a radio, you wanted to vary the notch above and below the passband. That made a fixed frequency notch something that nobody wanted ...Best guess is that a custom 455 KHz crystal (there are no standard parts anymore) will run you a couple hundred dollars. You then need to design up the filter and wind some coils to drop into it. Fire up the network analyzer and align the result. It can be done. I've done that sort of thing many times. It is a bit past the "solder in a part" sort of repair.Even with a "standard" filter, you still need to match the circuit impedances. If the source or load impedance is incorrect, the filter passband (or notch) will be wrong. On high order filters, a pretty small error will result in fairly large degradation. Even with a standard part, it's not a quick drop in.Bob

author: Tony_G
author_rank: Super Contributor
timestamp: September 01, 2016, 05:23:23 pm
content: Thanks Bob,I was thinking about just using a band pass filter to GND to achieve the same sort of thing.Given that the unit requires the front end RF amp to be repaired I don't really have much to lose by trying something and seeing what happens.I appreciate your response.TonyG

author: uncle_bob
author_rank: Unknown
timestamp: September 01, 2016, 05:42:24 pm
content: Quote from: Tony_G on September 01, 2016, 05:23:23 pmHiThat's not going to give you the response you are after. Filters are two port devices....It's not like a simple series tuned circuit.Any idea how narrow a notch you need and exactly what frequency it needs to be at?Bob

author: Tony_G
author_rank: Super Contributor
timestamp: September 01, 2016, 05:59:18 pm
content: Thanks again Bob - Appreciate the time you're taking to respond.I don't really have much information regarding the specific details of the amplifier and filter. From their perf tests I can work out that the gain should be flat across 2.5 to 1300 MHz at between 22.28dB to 24.82dB (tested at 11MHz).This is the bit in the service manual (Service Sheet BD2 8D-17):From reading the theory of operation I think it's there just top remove the noise from the IF filter when they're doing low power measurements. The frequency minimum for Tuned RF measurements in 2.5MHz so I don't think that the frequency/bandwidth of the filter is particularly important (but I don't actually know).TonyG

author: uncle_bob
author_rank: Unknown
timestamp: September 01, 2016, 06:19:01 pm
content: Quote from: Tony_G on September 01, 2016, 05:59:18 pmHiWithout the notch, the mixer passes the 455 straight to the IF (they don't just mix). Your sensitivity is degraded by the added noise. In addition, any IMD products that fall at 455 would be blocked by the notch. Best guess is that the notch is wider than the maximum IF bandwidth and likely 60 db down at that point. Depending on how wide the IF is, it could be just about anything in terms of construction. The IF could be 200 KHz wide .... The receiver *should* tune down to 150 KHz so there is some need for rejection.Bob

author: Tony_G
author_rank: Super Contributor
timestamp: September 01, 2016, 07:49:53 pm
content: Thanks again Bob,The receiver tunes frequency from 0.15 to 1300 MHz but the amplifier is only used for the lower range of the Tuned RF measurement as far as I'm aware. Tuned RF only works from 2.5 to 1300 MHz (according to the specifications in the manual).As long as the filter had something that was less than 2.5 MHz it should be OK to block the noise.TonyG

author: uncle_bob
author_rank: Unknown
timestamp: September 01, 2016, 08:38:02 pm
content: HiIf a $20 (maybe) high pass filter at 2.5 MHz would have done the trick, I don't see why HP would have spent (likely) $200 on a notch filter. They did go overboard sometimes, but not that far overboard.Bob

author: Tony_G
author_rank: Super Contributor
timestamp: September 01, 2016, 09:41:46 pm
content: Thanks again Bob - I really don't know, the only reference to the use of U1 I can find in the documentation is for the lowest range of Tuned RF measurement (per the image above). The filter is integrated inside the U1 package and not a separate thing. They might have been thinking about using this RF Amplifier in other designs or something.I understand what you're saying though - There has to be some other reason why they picked a notch filter instead of simply a high pass one for this particular application.I just don't know what that would be.TonyG

author: uncle_bob
author_rank: Unknown
timestamp: September 01, 2016, 10:01:24 pm
content: HiMy *guess* is that there is something about the receiver architecture that we are missing. I don't have one of these to play with so I can't really dig into that side of it.Bob

author: whollender
author_rank: Regular Contributor
timestamp: September 02, 2016, 05:52:33 pm
content: I took a quick look at the block diagrams in the service manual, and it does look like the notch filter is just there for noise rejection going into the mixer.I'd guess that they originally designed the amplifier path to support lower frequencies (<2.5MHz) than they claim in the manuals, so they needed a tighter filter than the 2.5MHz lower frequency limit would suggest. It may also have been easier to get the kind of rejection and higher frequency flatness they were looking for with a crystal based notch than a lumped element, or other kind of filter.

author: trukresom
author_rank: Contributor
timestamp: September 04, 2016, 10:37:22 am
content: As an owner of a HP8902A since a few weeks I am still in a kind of learning process with this instrument.So I was surprised to hear from Tony_G that there was some kind of notch filter in this RF preamp.Further I wanted to know if the preamp in my machine is working correctly. So I performed the test #1 ("10 dB Attenuator,RF Amplifier, and Amplifier/Switch Interconnect Check") on page 8F-20 of the service manual. First I had someproblems to unterstand table 8F-9. It is important to key in both special control function on one line in the indicated order.So to switch the amplifier on we have to key function 0.032 followed by 0.047.Once these tests showed me the correct levels at W8 I also measured the frequency response of the amplifier in therange DC-1MHz with a Rigol DSA815. The result is rather disappointing as shown on the attached picture.Can we really expect, that an attenuation of 4.5 db at 455 kHz is very useful ?Kurt

author: Tony_G
author_rank: Super Contributor
timestamp: September 04, 2016, 02:24:54 pm
content: Thanks Kurt - I didn't put a SA on mine because it was already operating too low - From the manual the range should be 22.28dB to 24.82dB - Mine was just below 22.28dB in gain.You can also use the procedure on page 8D-22 specifically to look at the input levels for W8 (table 8D-5) - You only need to hit 1.7SPCL to turn the amp on.It looks like yours will be in the middle of the range by the time you get to 11MHz which is used in the Front End Check test.BTW Would it be possible for you to do a sweep of the amp from 0.1 to 1300MHz? I'd love to see what a working amp's flatness looks like (so I can try and pick a replacement).TonyG

author: trukresom
author_rank: Contributor
timestamp: September 05, 2016, 07:45:08 am
content: QuoteGood point ! I didn't see this. It is indeed simpler to use these function codes.QuoteYes, of course.On pict1 you see the following:cyan trace: 0 dB gain (1.1 SPCL)magenta: +14 dB (1.8 SPCL)yellow: +24 dB (1.7 SPCL)On pict2:cyan: the test leads alone (connected on the end to verify the calibration)magenta: 0 dB (1.1 SPCL)yellow: -10 dB (1.2 SPCL)BTW: What is the primarily use of this receiver for you?I have not yet found any useful application for me. Even receiving any radio station does not work.Kurt

author: uncle_bob
author_rank: Unknown
timestamp: September 05, 2016, 12:19:28 pm
content: HiThe receiver is only useful for looking at noise. That is why the amp has a bunch of noise load specs on it ...Bob

author: Tony_G
author_rank: Super Contributor
timestamp: September 05, 2016, 02:42:57 pm
content: Thanks Kurt - I was using mine for checking & adjusting test gear and attenuators (till I dropped it off the bench.. doh!). As Bob said, it's not a receiver in the "listen to this radio" sense but more in the "I can see and analyze RF level signals that are produced by things" - It's the specified test gear for a lot of period HP signal generators and such (along with an 8903B Audio Analyzer).TonyG

author: uncle_bob
author_rank: Unknown
timestamp: September 05, 2016, 02:52:11 pm
content: HiOne big use for these was in EMI /EMC testing. That is the basis of my " listening to noise " comment.Bob

author: @rt
author_rank: Super Contributor
timestamp: October 05, 2016, 11:03:15 am
content: I have a 456kHz crystal I’m prepared to send.. baring in mind you should be able to drag it down a little below 456,and I’m guessing your filter probably wasn’t so tight that 1kHz would make too much practical difference.

author: Tony_G
author_rank: Super Contributor
timestamp: October 05, 2016, 02:19:39 pm
content: Thanks RT - Really appreciate that.I did further inspection of the unit once I decided that I wasn't going to repair it - I was thinking I could open the RF Amplifier up and see if the damage could be repaired. While doing that I saw that the board might have had some cracked mounts on it:I reflowed these and the system now doesn't show the primary error I was getting (Error 19 on the frequency drift test) so I'm planning on running it through the calibration tests again(need some kit at work to do that though and been rather busy). It also passes the Front End Check in BD2 (page 8D-22 of the service manual part 1) - It's on the low side but it is above the minimum - You can see the result of the test in this videohttps://youtu.be/9UZCGG2NP1g?t=8m17sHopefully I'll have the time in the next few days to test this out and will let you know how it goes.RT - I really appreciate the thoughtful offer, thanks for that.TonyG

author: @rt
author_rank: Super Contributor
timestamp: October 06, 2016, 04:49:47 am
content: I’m not sure wether that means you want it or notbut if so, pm an addressIt came from an avalanche tracker used by skiers, where in the finding mode,a pair of ferrite loop antennas are positioned 90 degrees from each other to compare amplitude of both signals to find direction.