are any SAW filters bidirectional? - Page 1
Forum section: electronics / rf-microwave

author: nali
author_rank: Frequent Contributor
timestamp: September 24, 2025, 03:34:39 pm
content: Without really thinking about it I assumed they were, but most DS distinctly specify a discrete input and output. Searching interweb manages to bring up both yes and no answersI've used a Qualcomm B3743 bandpass SAW in a 433.92MHz ISM receiver, but would like to make this a transceiver. So do bidirectional SAW exist, or maybe they're called something else? I'm limited in board space so a single-component solution would be ideal.

author: RadioNerd
author_rank: Regular Contributor
timestamp: September 24, 2025, 04:09:36 pm
content: As far as I know both the construction and specification of SAW Bandpass filters suggest a bidirectional operation like any other passive filter structure.The main limitation I can think of is the maximum allowed input power which is typically in the range of 10 dBm for standars SAW filters. BAW filters have a similar footprint but usually tolerate up to 20 dBm or even more. I am not sure whether they exist for 433 MHz, though.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: September 25, 2025, 02:13:19 am
content: A preamp with a 3710 SAW filter . I think the filters arebidirectional and I have made test boards for measurings-parameters, but I can't remember if I measured them.At the price of the SAW filters, I concluded that switchingcosts more & is too much ado, compared to 2 filters.The circuit ended up in my 70 cm transverter in DUBUS.BTW I measured 0.5 dB noise figure with my HP8970,DL7APV (RIP) measured 0.3 dB. I never found out wherethe difference came from, maybe the bandwidth of theanalyzer plays a role. The HP has 4 MHz BW; I can proveto myself that the SAW BW gives better noise results.I can also prove the contrary.73, Gerhard DK4XP(yes, I should really sleep now.)

author: nali
author_rank: Frequent Contributor
timestamp: September 25, 2025, 08:51:43 am
content: Looks like the answer is "Depends". FromMicrowave JournalQuoteQuotePart in question has 1.5dB insertion loss.I can move it so it's just in the RX path. I do need to add a bit of filtering on the TX side as the OOK IC does generate a bit of harmonic content, just would've been nice to run it through the same path (power level is low enough for the SAW to handle).

author: ejeffrey
author_rank: Super Contributor
timestamp: September 25, 2025, 02:40:04 pm
content: That doesn't make the SAW itself unidirectional. Ia simple SAW coupler launches and receives acoustic waves in both directions. You can think of it like a hybrid with the other port terminated. What they are talking about is engineering the transducer to only launch/receive accoustic waves in the direction of the other trasdicer. The device is still reciprocal.

author: nali
author_rank: Frequent Contributor
timestamp: September 25, 2025, 03:44:27 pm
content: Quote from: ejeffrey on September 25, 2025, 02:40:04 pmThat's exactly what the text is saying, and seems to apply to the part I have. That would explain why the ports are explicitly labelled as Input & Output (as is the case for all the other 433.92MHz SAW listed on Digikey)Quote from: ejeffrey on September 25, 2025, 02:40:04 pmI'm not in a position to reverse the part and test it. But compared to an almost identical design my signal is something like 40dB down, so it's as good as.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 25, 2025, 04:00:23 pm
content: Quote from: nali on September 25, 2025, 03:44:27 pmNah, they just label them as input and output so that they don't get customers calling their FAEs and asking "which side is the input"? The transfer function is guaranteed reciprocal.In principle there could be some difference in power handling, but I can't see why they would do that and every diagram and data sheet I have seen of a SAW filter is mirror symmetric.If they could make a passive isolater or circulator without bothering with bulky ferrites and magnets, I can tell you that you would be hearing about it. You wouldn't be trying to infer it from pin labels in a data sheet.

author: nali
author_rank: Frequent Contributor
timestamp: September 25, 2025, 08:07:04 pm
content: OK I can put this to bed now. I realised Icoulddo a reverse test as I had an old board with the same SAW that I could take off the radio IC and hack a SMA in its place and do a S21 sweep. You're right, there is no discernible difference between forward & reverse.What is causing the loss on the new board? That's a job for tomorrow, it's time for a beer now...