[time-nuts] 10 MHz Filters

Bob Camp kb8tq at n1k.org
Thu Nov 27 16:47:16 EST 2014


You can also discover interesting things about the transformer it’s self in some circuits. They do what they do, but they also bring along a new set of issues. Sometimes simpler is better. If you don’t *need* what the transformer does, why add more complications ? 

If you don’t lift the ground, the transformer isn’t doing much for you isolation wise.

If you do lift the ground, you need a lot more than just the transformer to make it all work and isolate properly. Once you add all that “stuff” you don’t have a simple circuit anymore.


One example:

The most likely “threat” to your distribution system is a cell or portable phone. The beast fires up at random times and spews RF around everywhere. That’s the signal you want to isolate and shield against. It’s at some frequency between 40 MHz and 6 GHz depending on what it is. What ever you do needs to work over that range for isolation / shielding. It also needs to pass 10 MHz. Don’t have a phone in sight? How about WiFi, Bluetooth, and RF based remote controls ….

Why is it a threat? The RF comes back into your amp and changes bias levels / inter modulates with the 10 MHz. Either way, your 10 MHz moves around a bit. Is this purely theoretical? Nope, it happens far more often than you’d think. I’ve seen it a number of times. 

A simple transformer at 10 MHz is not going to retain balance and isolation over 40MHz to 6 GHz. Even if it could the coax connector will imbalance it pretty badly. No balance = no isolation. 


Good cables and a passive splitter have their issues, so do logic gates. Both have their advantages. More complicated is not always better. 

23 dBm out of a KS box going down to +7 out gives you 16 db of room for attenuation. At 3 db for each 2:1 split, that’s 2^5 splits. It’s rare to find a need for 32 outputs. You can get 8 outputs at +13 dbm. That’s pretty hot for a distribution amp output.


> On Nov 27, 2014, at 2:02 PM, John Miles <john at miles.io> wrote:
> For the most part, you don't want transformer isolation unless you plan on using balanced lines.  There are worse things than ground loops out there, and lifting a coax shield away from ground is a great way to find all of them.
> You definitely don't want 10.7 MHz IF transformers, unless you are just trying to build a thermometer.
> -- john, KE5FX
> Miles Design LLC
>> -----Original Message-----
>> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Perry
>> Sandeen via time-nuts
>> Sent: Wednesday, November 26, 2014 8:08 PM
>> To: time-nuts
>> Subject: [time-nuts] 10 MHz Filters
>> List,
>> I have seen on the net a 10 MHz filter using 10.7 IFtransformers but have no
>> idea how well they would work for isolation with thenew style Lucent boxes.
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