A quick look at an Eiden 4220C-006 Broadcast Television Up Converter

More gear arrives from the world’s car boot sale – eBay. Once again there is no specification or documentation available.

For the past few months I’ve been writing about analogue television broadcast kit. So far it has mostly been related to sound modulation. In the background I’ve also been collecting other pieces, with the hope of re-creating a full sound and vision broadcast signal.

One the sticking points has been obtaining a suitable up-converter i.e. if I have a modulator with a 38.9 MHz IF (Intermediate Frequency) output, and for example I want a signal on (UK) UHF Channel 21 (474 MHz) – how do I do that exactly? Some modulators may have a fixed frequency up-converter built-in but this it not a given, and unlikely to be on the channel I actually want. I don’t even know what I want at this stage anyhow.

Could I build my own?

Sure I can throw together a quick setup using a mixer and an oscillator and I have done so on this page but the resulting signal also contains the local oscillator (LO) tone and the high side image of the carrier. No problem with a cable connection but these unwanted by-products have to be filtered out before on-air transmission and that is not an easy thing to do. Especially when we are talking about UHF frequencies. I’m not planning to put anything on-air (because it is illegal to do so) but still interested to see how this part is done.

A simple hairpin filter like the above could enable the construction of a DIY up-converter and would cost next to nothing to make. Figuring out the actual geometry of it however is quite another matter. Image credit: Charles Grassin

One of the most promising leads I found was here. This page talks about simulating hairpin filters in MATLAB – a lot cheaper than the ultra high-end software normally used to design these things. I gave it a whirl but found that even with the most powerful computer I have each simulation was taking about 15 minutes. Given that I’d need a lower frequency than he’s proven on his design, and steeper roll-off, this’d probably take me just about forever to get right. Very cool though.

Maybe not

Realistically this is a job for the pros. Let’s jump on eBay and see what’s out there.

From my searches the Japanese company Eiden come up as an example of a manufacturer of this final stage transmission kit, including the all-important upconverter. Rohde & Schwarz do too (they call them Transposers). Analogue-era broadcast upconverters are typically made for a pre-set channel because they contain a fixed SAW filter (newer units) or in the case of older units – large complicated filter networks machined (yes, from metal) for a particular frequency. They can also be Massive i.e. 6RU high and weighing 30-50 kilograms.

An enormous UHF Channel 50 up-converter for sale by the seller who I bought one of my pieces of NICAM kit from. Assuming the price, shipping and import taxes didn’t break my bank and the weight didn’t break my back, the next problem would be finding somewhere to put this monster.

A further complication is that while analogue telly is dead, the up-converters they used are still useful as digital television modulators are backwards compatible with these very expensive pieces of kit. For this reason most of what I could see for sale (including the above) is for decommissioned parts of the upper UHF spectrum where licenses have been revoked by governments and sold for 4G/5G cellular use.

Let’s have a look at this thing

Anyway. That is the big long ramble which explains why this thing is on my bench. There were a few of these 4220C units for sale and in particular one seller was open to my cheeky lowball offer.

The first thing to do is pull the lid off any see what’s inside. From initial examination it appears quite modern. Probably around 20 years old (newer than anything else I’ve looked at). It also looks like it cost tens of thousands of dollars.

It is very complicated as one would expect with the specification on offer. I’ll skip the detailed teardown and technical analysis of this unit. Its internals are not relevant to my endeavour at this time. Inside the aluminium modules are a step attenuator, mixers, oscillators, amplifiers and switchable filters.

There’s not much on the rear. Just the IF input and a shunt for the 2nd IF (used for >250 MHz operation). Not sure why you would want to disconnect this.

Operation

In a nutshell it upconverts the IF input to an output between 48-860 MHz. It’s final mixer is high-side spectrally inverting the signal as is required for television broadcast. I did quite a bit of testing on it and found that it gives a flat, clean output with an IF input anywhere from 29.5-54 MHz (assuming 474 MHz output) outside of this I started to see intermodulation products.

It is of a double heterodyne architecture with an optional low side mixer which upconverts the signal to around 280 MHz when dealing with a higher frequency output. Output filtering is exceptional. I wasn’t able to detect any modulation by-products on my spectrum analyser even 75 dB down.

This image captures how impressive this unit is. Upconverting a 36.15 MHz CW tone with a 0dBm output (full power) even down at -75 dBm there’s no detectable by-products. It does this across its full operating spectrum. Dang. No question this thing could be used for large scale broadcast.

Front panel / controls

Output frequency

The output frequency is selectable in 10 KHz increments. The frequency displayed on the front panel is the LO minus the IF centre frequency which can also be configured in a menu. In my case I’ve set it to 36.15 MHz. i.e. if my IF vision carrier is 38.9 MHz and I want it on UHF Channel 21 (471.25 MHz) I need a 510.15 MHz LO. I prefer to use centre frequencies so the front panel would then be configured to the UHF 21 centre frequency (474 MHz) plus 36.15 MHz = 510.15 MHz. Same same.

Power levels

The seller reckons it’s from -30 dBm to 0 dBm input. I guess that’s about right. I’ve been feeding it -10 dBm.

AGC

It has the option of automatic or manual gain control. Output configurable from -40 dBm to 0 dBm. It only has max about 20 dB of gain however.

Posted in Broadcast tech

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