Attenuators - theory and the design of.

[Argus25]

Lovely job on the PCB design !

[Wendymott]

Thanks Hugo. Really.. to make it wider bandwidth it should have a ground plane on top but this one is only for up to 30 Mhz...AND... I have a great deal of trouble getting registration.........top to bottom...I have some copper foil if I really need a ground plane.

[Wendymott]

Hi peeps... Further to the above..I had a few spare hours.. and thus I built a Mechanical switched attenuator adding an extra 20dB. All SMD resistors and the case is Bare pcb material. It was intended to be a replacement for the Teledyne switched attenuator in the SSB signal generator, but I had spaced the switches too close vertically, and not wanting to re drill the new front panel, I stayed with the original design and made this as well.

[Radio Wrangler]

The base of the teledyne relay is a ferrous disc with holes for the wires, so it looks like a multi-hole magnetic bead. it introduces series inductance and also series loss.

A colleague, Ian Matthews devised a compensation scheme to use them to 200MHz and above. One trip through the relay looks like 1.4 Ohms resistive plus I've forgotten how many nanohenries. He added shunt resistance and capacitance at each contact connection so the loss resistance became the series part of a low attenuation Pi attenuator section while the L term and the two shunt Cs added created a lowpass section. This improved flatness and return loss at the price of base attenuation and the rate of the eventual roll-off.

I later did an attenuator with Takemasawa or Midtex DIL relays where milled screening around each relay created distributed shunt-C to make the loops up to the contacts and back into 50 Ohm transmission lines.

At one time Teledyne's West coast facility burned down and its East coast facility got flattened by a hurricane. The US government responded by sequestering all stocks for their military whizz-bangs and oooh-narsties. Customers like HP were left to find alternatives as best they could.

David

[Wendymott]

Hi David. I had a Adret VHF Signal generator, which "died".. The Teledyne relays were taken from the attenuator. This was so heavily engineered, and I did note several custom made trimming caps at various points down the chain. At the time, I was not interested in keeping it as a "whole" item, thus it was recycled. However this version is not intended for lab use.. just my own.
RE availability of Teledyne..relays.. I bought some from a vendor on ebay recently at about £4 each, which I thought reasonable... they even had longer leads, compared with the adret items which were mounted on a very thin pcb, thus the leads were quite short.

[Argus25]

Nice work on the attenuator, it will be a useful device to have on hand.

I have used the Teledyne relays mainly as band change switches for my home brew MW-SW radios that go to 18MHz, so any lead inductance or capacitance issues were not a problem. I did this because I could use a miniature robust high capacitance industrial grade motor switch on the front panel. In my Fetron radio I used latching versions to save power and pulsed them via capacitors.

I found something here: there is significant cross coupling between the leads (in high impedance circuits), this caused feedback and oscillations in my high gain RF amp circuits, unless the body of the relay is grounded. So , not wanting to solder to it I made a spring 1 turn brass wire clip to earth the body. Also found the same issue with Fetrons,with a similar base, the body must be grounded.

I really like the Teledyne relay designs but obviously they start to have some limitations above 200MHz.

It always amazed me how they compressed the design of some into a TO-5 sized metal housing, and they always make plastic relays look cheap and nasty by comparison. I was surprised by the heat dissipation of a TO-5 type that I used to replace a faulty relay in a PM5519 pattern generator, so to be kind to it I put a TO-5 transistor heat sink on it, but it probably didn't need it.

[Wendymott]

Thanks Hugo. A marvel of engineering...the Teledyne relays. As I am not going anywhere near 200 Mhz..they are ok...
I used one in the VXO (9 Mhz) to switch upper / lower sideband, and six in the Sig gen attenuator.
I note the feedback issues if used in High "Z" circuits.