DIY bias-t - Page 1
Forum section: electronics / rf-microwave

author: zimm
author_rank: Newbie
timestamp: December 26, 2020, 07:16:11 pm
content: Hi.I need to power a GOES SAW filter but my SDRs don't have a bias-t option, so I'm trying to build one. Attached below is the schematic and PCB layout.The values for the components come from a picture I found of an old bias-t module, I'm not sure how to calculate them but the simulations I did in LTspice seem to indicate that they work for the frequency I'm interested in (1.7GHz). I'll put a screenshot at the end of the message.Since I don't really know what I'm doing, I have a few questions:is there a chance the schematic and component values will not work? maybe I did the simulation wrong...I heard/read somewhere that SMT components work better for RF stuff; is this true? will using through-hole components have a negative effect?I didn't do 50ohm impedance matching for the RF traces, should I?did I go overboard with the vias on the PCB? I found a plugin for KiCad which does via stitching and fencing and I was playing aroundAnd one last question, I have a HackRF which does have bias-t, but it's limited to 50mA. The filter I want to power needs around 180mA. Is there a (simple) way to increase the current provided by the HackRF?Thanks.

author: geggi1
author_rank: Frequent Contributor
timestamp: December 26, 2020, 07:36:23 pm
content: Some of the bias-T for TV usage can go up to this frequency range.

author: fourfathom
author_rank: Super Contributor
timestamp: December 26, 2020, 07:58:08 pm
content: For 1.7 GHz, you can reduce the capacitor and inductor values significantly. Try something between 100 nH and 1 uH for the inductor, and perhaps a 100 pF cap in parallel with a 0.01 uF. Make sure the self-resonant frequency of the inductor isn't below your operating frequency.Surface mount components are definitely a better idea at this frequency, and will let you shrink your layout a lot (also a good thing at this frequency). You might as well try for a 50 Ohm microstrip trace, necking down as needed to connect to the surface mount component pads. Don't be afraid to try surface mount, it's really very easy (especially with 1206 or 0805 or 0603 footprints). If I'm in a hurry I sometimes just cut traces on blank double-sided board, using an Xacto or a gouge. Edge-mount SMA connectors make it easy to hand-fab as well.

author: zimm
author_rank: Newbie
timestamp: December 26, 2020, 09:45:30 pm
content: I definitely want to try surface mount components, it's just that at the moment everything I have available is through-hole. I'm combing Ebay/Amazon/AliExpress for SMT kits to start with, any suggestions or links are much appreciated.For the inductor SRF, I looked it up and I'm a bit confused. I foundthis calculatorand when solving for frequency, putting in 0.01nF and 1uH gives me a resonating frequency of 0.05GHz, which is way below my frequency of interest... Am I missing something?

author: fourfathom
author_rank: Super Contributor
timestamp: December 26, 2020, 11:57:44 pm
content: All inductors have a stray parallel capacitance, often the inter-winding capacitance (each turn on the coil lays near another turn, and there is turn-turn capacitance). This stray capacitance means that an inductor, all by itself, forms a parallel-resonant circuit. The Self-Resonant Frequency is where the inductive reactance is equal to the stray capacitive reactance. Above the SRF, the inductor looks like a capacitor. The SRF is usually indicated in the inductor data sheet.Capacitors also have a stray series inductance, and so have a SRF as well. At the SRF they look like a low resistance, and above that they look inductive. It's all a bit more complicated than this (as is usually the case), but most of the time the simple description above is more than adequate.Your bias-T circuit should not be operating anywhere near the component's SRF, and the L and C here don't form a resonant circuit. The L is just a high impedance (more than 200 Ohms at the freq of operation), and the series C is just a low-impedance coupling element. The bypass capacitor on the power-input side of the L isn't critical, and also not in any tuned circuit.You might look at domestic distributors for the inductors (I use Digikey) since at this frequency you really ought to look at trustworthy component specs. The capacitors can be more generic and available cheaply in kits, but do get "COG" for the low-value ones. How you get your parts will depend on what your planning for. For SMT capacitors I ended up buying a full set of the "20%" series between 10pF and 10nF, and a scattering of higher and lower values. For inductors I got a few values between 100nH and 10uH, and the rest I can air-wind or use toroid cores (I have an assortment of those). This suits my RF experimenting needs.While I prefer SMT for this stuff, you can certainly use through-hole components for your bias-T. You can wind an air-core inductor on a 1/8" form, eight turns, 1/2" long -- this gives you about 0.08uH, which is more than enough for your T. Here's a coil calculator link:https://www.66pacific.com/calculators/coil-inductance-calculator.aspxI honestly couldn't tell you what the SRF of such a coil might be though.

author: rfclown
author_rank: Frequent Contributor
timestamp: December 27, 2020, 12:04:48 am
content: I concur with fourfathom. I often use an exacto to cut microstrip lines in bare PCB and solder edge mount SMA connectors. Since I do this so often, earlier this year I bought this from eBay:https://www.ebay.com/itm/201837202416It's 8 PCBs with just a 50 ohm line and SMA connectors. For something simple like a bias tee, I'll cut one of these boards in half. The connectors with this eBay item are really cheap (not great connectors), but they work. Also, I don't think the line is exactly 50 ohms, but for 1.7 GHz it's ok. I tried something at 6 Hz with these, and it wasn't the best.I made a bias Tee earlier this year using one of these boards for a similar application as yours. I suspect you have a NooElec LNA since my first impression when I read your post was, "180mA! Who needs that much current for a filter!". Then I remembered that I experimented with a similar NooElec LNA this last year. I used the following components for my bias tee:1. For the DC block (your C1/C2), I used a 33pF 0603 cap. I used this value since it was near resonance at 1542 MHz.251R14S330GV4T Johanson Technology CAP CER 33PF 250V C0G/NP0 0603 High-Q712-1341-1-ND Digikey CAP CER 33PF 250V C0G/NP0 0603I have a kits of Johanson parts, which is why I chose this particular one. You want C0G/NPO caps for this part. NPO means that the value doesn't vary over temperature. Look at the manufacturer's datasheet to see where the self resonance of the part is. Normally you want to stay below self resonance. For an RF DC block, you can play a trick and use the part at self resonance to reduce losses. You can't do this for a broadband application. You can just cut a gap in the microstrip line, and solder the cap across the gap. This is where SMT is way easier than through hole.2. For the RF choke (your L1) I used a 33nH 0603.LQG18HN33NJ00D Murata IND 33nH 500mA 0.54Ohms 5% 0603 Active81-LQG11A33NJ00 Mouser Fixed Inductors 0603 33nH 5%490-6580-1-ND Digikey FIXED IND 33NH 300MA 600 MOHMWhere I was at the time I selected this, they had kits of these Murata parts. I had my Johanson kits at home.Again, look at the manufacture's datasheets for self resonant, current rating, DCR (resistance) info. You want to be below self resonance here.3. For the DC bypass (your C3), I used a 0.01uF (10 nF) 0603.I used an exacto to cut a rectangular pad at the edge of the PCB to attach the DC wire. Soldered C3 from there to ground. Cut a gap in the microstrip line, soldered C1 across it. Tombstone soldered L1 to the microstrip, and soldered a thin wire from the other side of L1 to the DC pad. Not pretty, but I made it in no time at all, and it looked great on a Network Analyzer.

author: rfclown
author_rank: Frequent Contributor
timestamp: December 27, 2020, 12:45:30 am
content: I'm NOT recommending this particular seller. I just went to eBay and typed SMT, and this is the first that popped up.https://www.ebay.com/itm/0201-0402-0603-0805-1206-SMD-SMT-Capacitor-Chip-Resistor-Samples-Book-Kit-US/203037501968?hash=item2f45fa6a10:g:cjIAAOSwiC1e~YzKBut it looks similar to one's I've bought. The first one I bought (several years ago) was an 0402 resistor kit. Then later I got a 0603 resistor kit. A couple years ago I got one that had a mix of resistors and capacitors.But for RF stuff, I'm not usually reaching for these kits. I do this for a living, so I get manufactures giving me SMT cap and inductor kits. They don't seem to do the same for resistors, which is why I bought the eBay resistor kits. For common values, I go to Digikey and get 50 or 100 on Cut Tape. 0 ohm, 1k, 10k, 100pF, 1000pF, 0.01uF, 0.1uF, 1uF, etc. Whatever you use alot of. I like Digikey because they tend to have anything your looking for, and if it's not heavy, they'll ship USPS first class and shipping isn't expensive. I use Mouser next if I don't find what I want on Digikey. Digikey's search is the bomb.The bias tee I described earlier isn't here with me now, so I looked at my stash a found one I'd made years ago, i think for GPS. But you see how nicely SMT parts work with microstrip and simple board layout. This board wasn't laid out specifically to be a bias tee. It was just a 50 ohm line with provision for a DC feed. I would use the board for filters and such, cutting the microstrip where needed.

author: cdev
author_rank: Super Contributor
timestamp: December 28, 2020, 02:01:53 pm
content: @rfclownWhere can somebody get your PCB material you carved up there with the small number of vias predone?Do you just chop up scrap boards?That ebay vendor's boards are just too expensive, IMHO.

author: rfclown
author_rank: Frequent Contributor
timestamp: December 28, 2020, 06:00:02 pm
content: I did those boards at a former job like 15 years ago. They had an in house prototype PCB lab. I made a panel of boards with just a 50 ohm microstrip and provision for bias on one or both ends. I actually used ADS! No schematic, just pushing polygons around in the layout tool. I have a few that took with me when the lab closed. The Greek guy's auction seems expensive, till you add it up. Your get 8 boards with connectors. That's $42/8 = $5.25 per board. Figuring the cost of the connectors, that's a cheap board. I'm not cash strapped, so for me it's the time. I find myself making little one-off circuits all the time, so it's handy to have these around.The following board house is inexpensive:https://oshpark.com/There was a board done in Eagle at work by a guy who left the company, and I was asked to order more boards. I uploaded the eagle file on the oshpark website, and it was like $15 for 50 VERY small boards. I need to learn KiCAD like you. I downloaded it this year, but haven't used it. At work we have full time layout people, so I haven't had to do my own layout for many, many years. Just schematic. My present job uses PADS, and I'm never going to have that at home. I don't like how Eagle has gone to the Web based thing. I did a few boards with it about 10 years ago. So I find myself hand making boards using an exacto knife and a dremel tool with a fine diamond tip. For vias I use a small drill then solder a wire top and bottom. Then using a nibbler to cut the board (which really wastes board material). The Greek guy's stuff for me is just a time saver.

author: rfclown
author_rank: Frequent Contributor
timestamp: December 28, 2020, 09:14:42 pm
content: For your other question about the HackRF, the bias tee current can be limited by either the power supply feeding the tee, or the RF choke. It's much easier and cheaper to find a good inductor for an RF choke (high self resonant frequency) if the current requirement is small. For higher current, the winding wire needs to be bigger, and the parasitic capacitance between the windings is more prominent. For the HackRF, I Googled HackRF bias tee and found:https://www.rtl-sdr.com/forum/viewtopic.php?t=4947So.. with HackRF, it's both problems. The supply is from USB, which has to power HackRF and this external bias. And whatever inductor they chose (L3, I didn't look it up) isn't rated for much current. If it was ONLY the inductor, you could find a different one to use; but since it's also a pass transistor (Q4) and an issue with USB, you're better off using an external tee.

author: thinkfat
author_rank: Unknown
timestamp: December 28, 2020, 10:28:05 pm
content: A Bias-T is about the simplest thing you can make, unless you need to transfer a lot of power.Not expensive to make, either, but $5 is not too much, I'd say.2 * SMA edge connector, about $1 (if you buy cheap Chinese via LCSC)1 * 27nH inductor, about $0.80, Johanson via Mouser1 * 33pF NP0 MLCC, about $0.10, AVX via Mouser1 * 10nF NP0 MLCC, about $0.20, Murata via MouserMaybe you want to add a small resistor in the DC feed for current limiting.PCB: < $1 for a 4 layer PCB.Raw BOM: about $3.Adding shipping cost for the parts, it will not make sense to build only one. So you make, like, 20? 50?It'll cost time to assemble them. No, $5 a piece is not too much. You can only be "cheaper" if you already "have" the parts.PS: you will want to make a 4-layer PCB. Even if you don't need it routing-wise, you'll hardly find a cheap PCB maker that will offer impedance control for a two layer PCB. 2GHz is not crazy-high but you'll still want a known-to-work stack-up.Another thing, you'll want to look at how other people do RF layouts. What you've shown above likely won't work for 2GHz.Also, you'll want to invest in a cheap VNA. Get a NanoVNA V2, for example, will not cost you a kidney and make your life a lot easier.