一个VHFFM放大器的几点疑问
是一个老外的。
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VHF FM Antenna Booster Circuit
Together with a good directional antenna this high performance two-stage antenna amplifier for the VHF FM broadcast band will enable you to capture far removed (DX) stations. Alternatively, it will drastically improve reception of FM signals you’ve come to accept as marginal and noisy in your area.
Together with a good directional antenna this high performance two-stage antenna amplifier for the VHF FM broadcast band will enable you to capture far removed (DX) stations. Alternatively, it will drastically improve reception of FM signals you’ve come to accept as marginal and noisy in your area.
成熟的人的表现FM/VHF antenna amplifier circuit schematic
最懂我的那个人600字作文For various reasons, an increasing number of people are not satisfied with the quality of radio signals that can be received via cable systems. Unfortunately, cross-modulation, and other nasty effects created in the head end station are in stark contrast with the superb quality of high-end FM tuners that can be bought commercially.
Not surprisingly, owners of such tuners will often prefer to have their own antenna on the roof. Add to that a high-performance FM antena booster like the one described here and you can start DXing as well as enjoying high-quality stereo reception. The author employs the prent amplifier in combination with a Sangean ATS-803 World band receiver, a Philips RR-571 tuner and a single-element cubical quad directional antenna at a height of just 15 feet. Using this tup he is able to receive FM stations as far off as 500 km.
Antenna booster circuit description
In the design we’re about to describe you’ll find the following important parameters coupled: low noi figure (approx. 1 dB); high gain (up to 40 dB) and low susceptibility to i
ntermodulation products. Yet the amplifier is inexpensive and easy to build. The circuit diagram in Figure 1 reveals the cret: dual-gate MOSFETs in positions T1 and T2. The first of the, T1, is configured mainly for low noi and antenna matching and the cond, T2, for high gain. Unusually, the antenna signal is applied to T1 via its source (S) terminal, which is convenient becau unlike one of the gates (G1 and G2) it already reprents a fairly low impedance. None the less, for impedance matching with the 50 Ω coax cable a tap needs to be ud on tuned filter L1/C2.
红头文件制作For VHF and UHF amplifiers, DG-MOSFETs reprent a good alternative to cheap but noisy bipolar transistors and very expensive and difficult to get Ga-As FETs.
The gain of the cond DG-MOSFET in the circuit is adjustable using pret P1 which varies the bias voltage to T2’s G2 terminal — this is the classic way of controlling the gain of a DGMOSFET and it still works very well. Such a control is included in the design to allow you to get the exact amount of gain required for your particular application. For example, if you live near a powerful VHF FM or TV broadcast transmitter then you’ll find t
如何增发hat a lot of gain produces cross-modulation and other unwanted effects like coupled oscillation and 'birdies’ within the FM band.
For stability the supply voltage is decoupled at veral locations by electrolytic capacitors and ceramic capacitors for the low and high frequencies respectively. To enable it to be mounted as clo as possible to the antenna, the amplifier is powered via the download coax cable, i.e., over coax connectors connected to K2 and K3. In the supply, the RF signal is taken off the coax core by capacitor C19.
The supply voltage is adjustable to some extent with pot P2, which will also allow a degree of gain control. The current through 'power on’ LED D2 should be about 10 mA. If a low-current LED is ud, then R8 has to be incread accordingly. With electrical safety in mind we strongly suggest the u of a mains adapter with 12 VDC output. Depending on the gain t and the DG-MOSFETs ud the circuit will consume about 50 mA so a 100-mA or slightly more powerful adapter will be fine in most cas.
VHF FM antenna amplifier construction
毛泽东作品The amplifier is built on the singlesided printed circuit board shown in Figure 2. The inductors in the design are all very simple to make, e the parts list for construction details. L1, L2 and L3, are wound on a 4.5-mm dia. drill or pencil. L1 then needs to be stretched to a length of about 10 mm. You’ll find that a relatively large resistor is needed to wind choke L4 on — we ud a 0.5-watt carbon-film resistor from the junkbox (metal film resistors em to have taken over completely). The value 1 MΩ is uncritical, what we’re after is 'a lot’ of carbon for the core so 820 k or 1.2 M will do just as well.
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FM antenna booster PCB layout
The DG-MOSFETs are not only nsitive to static discharges but also easy to fit the wrong way around. The non-SMD versions require 5-mm holes t be drilled in the PCB. Make absolutely sure you know each MOSFET’s final orientation on the board before soldering it in place — check, think hard and refer to the component drawing shown in Figure 1. You may find that the legs are a bit too long for the PCB but don’t u your cutters until the very last moment becau without the stud marker (source terminal) you will be lost for device orientation.
Several VHF/UHF DG-MOSFETs may be ud in this circuit — e Table 1. With some dexterity it is also possible to u SMD devices. In general, you should aim to u a BF9xx with a low noi figure, although that may imply a slightly reduced overall gain. Remember, however, that the amplifier’s gain is condary to the noi figure — in practice, any gain between 25 dB and 40 dB will be just fine.
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