High-resistance low-noise preamplifier at 144MHz S53WW

Robert S53WW je skvelý konštruktér zariadení na vkv. Na jeho stránkach nájdete napríklad aj popis sústavy antén na 2m vhodnej do contestov a kvalitného transvertoru Javornik 144/14MHz. Poprosil som ho o súhlas na preklad a publikovanie týchto článkov. Robi mi ho obratom zaslal, za čo mu patrí veľké ďakujem. Postupne Vám ich všetky prinesieme na www.CQ.sk But if you don't want to wait, visit his website http://lea.Ham radio.si/~s53ww/.

Introduction

After a thorough analysis of various preamplifiers for my new 144/14 MHz transverter I ended up staying with the Infeon BF998 MOS-FET. I had a very good experience with the old BF-981, so I started testing the BF998. This transistor is cheap, easily available and has very good characteristics. I didn't use GaAs FETs because they are hard to come by and expensive, and the differences are minimal.

It was clear from the analysis of the transverter that it determines the final resistance of the entire assembly preamplifier (if you use a +17dBm mixer with a +20dBm oscillator signal and behind it an amplifier with IP3 out > 36dBm and neglect KV resistance TCVR)! OK, commercial KV receivers should not have poor resistance (IP3 in < +15dBm to +25dBm, AOR-7030 to +30dBm). In this case, IP3 out should be at least at this level (+30dBm). The noise of KV receivers is at the level of 16dB, in the best case around 13dB.

If we estimate the loss in the ANT-XVRT coaxial cable to be about 0.5dB and require a resulting noise figure of around 2.2dB, we find that the XVRT gain must be at least 24dB (with a noise figure of 1dB). It follows that IP3 in must be more than +6dBm (=30dBm-24dB). It is not easy to meet this requirement precisely because of the preamplifier.

Low-noise preamplifier with parallel combination n times BF998

An attempt to fit the original preamplifier with a new BF998 did not bring significant improvement. He made a profit 26.5 dB, noise number 0.8 dB, P1dB 17 dBm and IP3 in 0 dBm. They are not bad values, but they could have been better.

The first picture shows a common connection of a preamplifier with a two-base MOSFET with maximum profit in mind. Idss is set to the minimum noise level (Idss is given in the range of 2 to 18mA - usually it is between 10 to 15mA). Input coil L1 is wound on a diameter of 5mm with 1.0mm AgCu wire; it has 6 turns with a winding length of 11 mm. It should be placed at least 2 mm above the ground and at least 10 mm from the metal parts of the box. Capacitance trimmers must be of good quality. Diodes (Infineon BAR63-04 or BAR64-04 or BAR14-1) serve to protect the MOSFET. They have almost no effect on the noise figure or resistance, their consumption in real conditions is desirable. The most critical part is transformer TR1: should be wound on a two-hole core of size A7 made of U17 material (Epcos, formerly Siemens&Matshushita). Materials from other manufacturers may be used with respect to gain degradation and P1dB. We wind two turns trifilarly - it is always better than an autotransformer with 6 turns (=2×3) of the primary winding and a branch on the second turn from the cold end (in practice, 4 turns are wound first, a branch is created by twisting the wires, and 2 more turns are wound). The TR1 leads must be shortened to 5mm and routed near the ground on the printed circuit board, otherwise oscillations at high frequencies may occur.

While experimenting, I found an interesting FET connection (used in NF amplifiers), where two identical FETs connected in parallel have lower noise than one! So if you select another BF998 (must have the same Idss, must be selected), you can get up to the value 0.6dB NF, profit 26.5dB and the same or worse (!?) IP3 in. The problem with IP3 can be solved by adapting the output more appropriately - instead of a transformer with a ratio of 3:1, we will use a 2:1. In this way, we achieve +28.5dBm of IP3 power (P1dB is +19dBm). The general turn-on of the LNA n times BF998 is in the next picture (the achieved values ​​are listed in the table).

Of course, I also tried four BF998s (who wouldn't) - the noise figure is reduced to 0.5dB, the profit remains 26.5dB (when set to minimum noise figure) and IP3 out is +34dBm. To achieve this IP3 value, it is necessary to change the TR1 ratio to 1.3:1. Branch L1 is in the middle of the winding (on the third turn). At the first attempt, the amplifier oscillated above 3.5GHz, but this did not affect 2m. I tried placing a pair of transistors on each side of the PCB connected by short wires and separate G2 bias circuits (layout of components and blocking capacitor is critical). After this modification, the oscillation disappeared, but I would like to note that all connections are stable only under certain conditions. The layout of the parts is critical, especially for the 1nF blocking capacitor.

Circuit diagram of preamplifier n times BF998 (similar to single BF998). The values ​​of the variable components can be found in the table. The adjustment of input and output is also different (see text). The material of the transformer is the same as in the previous case, when using several BF998 I recommend A4 size. We can tune the preamplifiers without measuring technology with a small trick to the minimum noise figure: we tune them to the maximum gain at 136 MHz (the noise figure hardly changes in a wide range).

You can download the circuit diagram here ftp://lea.Ham radio.si/pub/vhfct/lnaa2.zip.

Push-pull LNA with BF998

The push-pull amplifier is comparable in gain to a two-stage simple amplifier, and the IP3 is about 3dB better than the parallel design. So I tried to connect a pair of BF998 in push-pull. The first results were excellent: IP3 was +6 dBm (P1dB +23 dBm), but the noise figure was at least 1.3dB! After changing the input circuit, I achieved the noise figure 0.9-1.0dB, IP3 out 32.5dBm at a profit 26.5dB (set to minimum noise figure). The resulting connection is the connection of two single-stage amplifiers according to the first picture.

Push-pull connection of two BF998. The input coil L1 is wound on a diameter of 11 mm with a wire of 1.0 mm AgCu; it has 4 turns and the winding length is 7mm. The coupling coil has 1 turn on the same diameter as L1 enameled Cu wire so that it cannot short out L1. Transformers TR1 and TR2 are on the same material as a simple LNA (size A7, material U17, Epcos). The output transformer TR2 has two bifilar windings.

Then I tried the push-pull connection of four BF998. The intention was to get IP3 in around +9dBm with a noise figure of about 0.8dB. Even though I achieved IP3 in as expected, it was not in my power to achieve the desired noise figure. This was still higher than 1.3dB, which I found inappropriate for the XVRT. However, this value may be suitable for other purposes, it still requires experimentation.

Notice:
These preamplifiers are not suitable for direct connection in front of the receiver or transverter because they have a very high gain. Their effective use (although I do not recommend using preamplifiers) is possible with an attenuation element at their output, which will reduce the gain. In most cases 6-10dB of gain is sufficient for a preamplifier if it really has to be used. With a -20dB attenuation element, the overall noise figure deteriorates from 0.5/0.6/0.8dB to 1.3/1.4/1.5dB. With a -16dB attenuation element, the overall noise figure deteriorates from 0.5/0.6/0.8dB to 0.8/0.9/1.1dB. So I recommend you to use a -16dB attenuator after the preamplifier (the values ​​of the pi-cell are 68 ohms to ground and 150 ohms between them).

For high quality VHF and higher preamps, check out Dragan's YU1AW site: www.QSL.net/yu1aw/low_noise.htm.

By September 2002, I had built six preamplifiers with a noise figure of 0.5-0.6dB (BF998 Philips, labeled Mop) and three with a noise figure of 0.8-0.9dB (BF998 Infeon, labeled Mos). The bad news is that Epcos has stopped distributing U17 material and it is very hard to get U17/A4 dual hole cores. Their replacement is not yet known, but cores of a similar size made of material with a permeability between 20-30 are usable...

After measuring nine preamplifiers, I recommend a simple way to adjust the LNA 4xBF998 to minimum noise: tune the trimmer C to maximum profit, then increase its capacity until profit does not drop by 1dB! Tuning is not critical, the input circuit is broadband when setting the minimum NF (about 10 MHz).

Robi, S53WW
http://lea.Ham radio.si/~s53ww/
Translated and adapted with the permission of the author Viliam, OM3-0122